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21 5 N. RANDALL A, ■:

by Google

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INTERNATIONAL
LIBRARY OF TECHNOLOGY

A SERIES OF TEXTBOOKS F6R PERSONS ENGAGED IN THE ENGINEERING

PROFESSIONS AND TRADES OR FOR THOSE WHO DESIRE

INFORMATION CONCERNING THEM. FULLY ILLUSTRATED

AND CONTAINING NUMEROUS PRACTICAL

EXAMPLES AND THEIR SOLUTIONS

DESIGN OF ALTERNATING-CURRENT

APPARATUS

ELECTRIC TRANSMISSION

LINE CONSTRUCTION

SWITCHBOARDS AND SWITCHBOARD

APPLIANCES

POWER TRANSFORMATION AND

MEASUREMENT

SCRANTON:

INTERNATIONAL TEXTBOOK COMPANY

13B

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Copyrififht, 1906. by International Textbook Company.

Entered at Stationers' Hall, London.

Desigm of Alternatkifir-CaiTent Apparatus: Copyright, 1905, by International

Textbook Company. Entered at Stationers' Hall, London.
Electric Transmission: Copyright. 1905. by International Textbook Company.

Entered at Stationers' Hall, London.
Line Corjtriliction: Copyrigrht, 1906, by International Textbook Company.

Entered at Stationers' Hall, London.
Switchboards and Switchboard Appliances: Copin^srht. 1905, by International

Textbook Company. Entered at Stationers' Hall, London.
Power Transformation and Measurement: Copyright. 1905. by International

Textbook Company. Entered at Stationers' Hall, London.

Printed in the United States.

//18b

burr printing house,

frankfort and jacob streets,

NEW YORK. 219

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104378

MAR 3 0 1S07

\1>

PREFACE

The International Library of Technology is the outgrowth
of a large and increasing demand that has arisen for the
Reference Libraries of the International Correspondence
Schools on the part of those who are not students of the
Schools. As the volumes composing this Library are all
printed from the same plates used in printing the Reference
Libraries above mentioned, a few words are necessary
regarding the scope and purpose of the instruction imparted
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these Schools, in order to afford a clear understanding of
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The only requirement for admission to any of the courses
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plete in itself, and no textbooks are required other than
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selected. The students themselves are from every class,
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almost without exception, busily engaged in some vocation,
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outside of their regular working hours. The information
desired is such as can be immediately applied in practice, so
that the student may be enabled to exchange his present
vocation for a more congenial one, or to rise to a higher level
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iii

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iv PREFACE

In meeting these requirements, we have produced a set of
books that in many respects, and particularly in the general
plan followed, are absolutely unique. In the majority of
subjects treated the knowledge of mathematics required is
limited to the simplest principles of arithmetic and mensu-
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matics needed than the simplest elementary principles of
algebra, geometry, and trigonometry, with a thorough,
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To effect this result, derivations of rules and formulas are
omitted, but thorough and complete instructions are given
regarding how, when, and under what circumstances any
particular rule, formula, or. process should be applied ; and
whenever possible one or more examples, such as would be
likely to arise in actual practice — together with their solu-
tions— are given to illustrate and explain its application.

In preparing these textbooks, it has been our constant
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correct any and all ambiguous expressions — both those due
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or explanation. As the best way to make a statement,
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diagram in connection with it, illustraticms have been used
almost without limit. The illustrations have in all cases
been adapted to the requirements of the text, and projec-
tions and sections or outline, partially shaded, or full-shaded
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produce the desired results. Half-tones have been used
rather sparingly, except in those cases where the general
effect is desired rather than the actual details.

It is obvious that books prepared along the lines men-
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PREFACE V

indexes are so full and complete, that it can at once be
made available to the reader. The numerous examples and
explanatory remarks, together with the absence of long
demonstrations and abstruse mathematical calculations, are
of great assistance in helping one to select the proper for-
mula, method, or process and in teaching him how and when
it should be used.

The first portion of this volume contains an exceptionally
distinct and intelligible treatise on the complex problems
relating to the design of alternating-current apparatus. The
correct proportions and relative location of the different
parts of the machines are clearly set forth and illustrated by
numerous figures showing the details of the construction.
The design of alternators, motors, and transformers is fully
discussed. The various systems of transmitting electrical
energy, and the methods used in calculating the size of wires,
and installing the wires for overhead and underground trans-
mission systems, are described in great detail, and complete
wire data tables are furnished. The treatment of switchboards
in this volume is very complete and is superior to anything yet
published. The recent styles of oil switches, circuit-breakers,
measuring instruments, etc. are fully explained and illustrated,
and their location indicated on the switchboard diagrams.
Under the heading Power Transformation and Measurement,
a very clear treatise is given of the installation of transform-
ers and substations and the methods of power measurements.

The method of numbering the pages, cuts, articles, etc.
is such that each subject or part, when the subject is divided
into two or more parts, is complete in itself; hence, in order
to make the index intelligible, it was necessary to give each
subject or part a number. This number is placed at the
top of each page, on the headline, opposite the page number;
and to distinguish it from the page number it is preceded by
the printer's section mark (§). Consequently, a reference
such as § 16, page 26, will be readily found by looking along
the inside edges of the headlines until § 16 is found, and
then through § 16 until page 26 is found.

International Textbook Company

I3-B

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CONTENTS

Design of Alternating-Current Appa-
ratus Section Page

Alternators 20 1

Limitation of Output 20 2

Heating of Alternator Armatures .... 20 4

Relation Between/*^ Loss and Output . 20 6

Core Losses 20 7

Hysteresis Loss 20 7

Eddy-Current Loss 20 9

Radiating Surface of Armature 20 10

Armature Reaction 20 11

Armature Self-induction 20 15

Peripheral Speed of Alternator Arma-
tures 20 20

Armature Windings 20 21

Single-Phase Concentrated Winding ... 20 22

Single-Phase Distributed Windings ... 20 23

Polyphase Armature Windings 20 27

Arrangement of Windings 20 29

Construction of Armatures 20 31

Armature Disks 20 31

Armature Spiders 20 34

Armature Conductors 20 38

Forms of Armature Coils and Bars ... 20 39

Armature Insulation (Coils) ...... 20 42

Armature Insulation (Slots) 20 43

Magnetic Densities 20 46

Density in Armature Teeth 20 46

Density in Armature Core '^ '^ . 20 47

iii

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iv CONTENTS

Dbsign of Alternating-Current Appa-
ratus— Continued Section Pagi

Density in Air Gap 20 48

Desigfn of 100-Kilowatt Single-Phase

Alternator 21 1

Dimensions of Conductor and Core ... 21 3

Design of Armature Core 21 4

Calculation of Armature Losses 21 10

Armature Winding for Two-Phase Alter-
nator 21 13

Armature Winding for Three-Phase Alter-
nator 21 15

Completed Armatures 21 19

Design of Field Magnets 21 20

Revolving Fields 21 23

Field-Magnet Coils 21 25

Insulation of Field Coils 21 27

Design of Field 21 28

Bore of Poles and Length of Air Gap . . 21 28
Magnetic Flux Through Pole Pieces and

Yoke ... 21 30

Calculation of Field Ampere-Turns ... 21 32

Calculation of Separately Excited Winding 21 34

Compound, or Series-Field, Winding . . 21 38

Loss in Field Coils 21 42

Mechanical Construction 21 43

Field Frame and Bed 21 43

Collector Rings and Rectifier 21 45

Brushes and Brush Holders 21 50

Brush-Holder Studs 21 51

Shafts 21 54

Pulleys 21 55

Connections 21 57

Transformers 22 1

Transformer Cores 22 4

Heating of Transformers 22 4

Magnetic Density in Core 22 5

Arrangement of Coils and Core .... 22 6

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CONTENTS V

Design of Alternating-Current Appa-
ratus— Continued Section Page
Winding and Insulation of Coils .... 22 8
Design of 8-Kilowatt Transformer ... 22 10

Determination of Core Volume 22 11

Dimensions of Core 22 12

Dimensions of Conductors 22 13

Calculation of Primary and Secondary

Turns 22 15

Arrangement of Primary and Secondary

Coils 22 16

Efficiency 22 19

Efficiency Curve 22 21

All-Day Efficiency 22 23

Magnetizing Current 22 24

Regulation 22 25

Construction 22 27

Induction Motors 22 30

Limitation of Output 22 31

Primary Core Losses, Magnetic Densities,

Etc 22 31

Secondary Core Losses, Magnetic Den-
sities, Etc 22 32

Induction-Motor Windings 22 33

Primary Winding 23 33

Secondary Winding 22 35

Power Factor 22 36

Length of Air Gap .22 37

General Data .22 37

Design of 10-Horsepower Motor .... 22 40

Full-Load Current in Primary 22 41

Size of Primary Conductor 22 42

Peripheral Speed and Diameter of Arma-
ture 22 42

Primary Winding 22 43

Magnetic Flux in Poles 22 45

Secondary Winding 22 50

Rotary Conductors and Core 22 50

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vi CONTENTS

Design of Alternating-Current Appa-
ratus— Continued Section Page

Heat Losses 22 52

Field Winding and Connections 22 55

Mechanical Construction 22 56

Armature 22 56

Shafts 22 56

Field Frames and Bedplate 22 57

Electric Transmission

Introductory 23 1

Power Transmission by Direct Current . 23 2

Line Calculations 23 7

Power Transmission by Alternating Cur-
rent 23 23

Single-Phase Transmission 23 24

Two-Phase Power Transmission 23 26

Three-Phase Power Transmission .... 23 28

Line Calculations for Alternating Current 23 30

Formulas for Line Calculations 23 31

Selection of a System 23 36

Direct-Current Systems 23 36

Alternating-Current Systems 23 39

Cost of Conductors 23 43

Combined Operation of Direct-Current

Dynamos 23 45

Operation of Dynamos in Series .... 23 45
Operation of Direct-Current Dynamos in

Parallel 23 45

Combined Running of Alternators .... 23 58

Alternators in Series 23 58

Alternators in Parallel 23 58

Line Construction

Introduction 24 1

Line Conductors 24 1

Overhead Construction 24 14

Cross-Arms 24 16

Pins 24 19

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CONTENTS vii

LiNB Construction — Continued Section Page

Tying, Splicing, Etc 24 23

Underground Construction 24 32

Conduits 24 33

Manholes 24 38

Edison Underground Tube System ... 24 53

Tests 24 58

Testing Lines for Faults 24 58

Switchboard^ and Switchboard Appli-
ances

Switchboard Appliances 25 1

Switches . 25 1

Bus-Bars 25 19

Fuses and Circuit-Breakers 25 27

Ground Detectors 25 36

Potential Regulators . 25 42

Protection From Lightning and Static

Charges 25 47

Field Rheostats 25 65

Switchboards 25 71

Direct-Current Switchboards 25 73

Alternating-Current Switchboards .... 25 76

Power Transformation and Measurement
Transformers and Transformer Connec-
tions 26 1

Transformers on Single-Phase Circuits . 26 4

Transformers on Two-Phase Circuits . . 26 9

Transformers on Three-Phase Circuits . . 26 11

Substation Equipment 26 18

Apparatus for Controlling the Incoming

Current 26 20

Apparatus for Transforming the Current . 26 26
Apparatus for Controlling the Outgoing

Current 26 40

Location and General Arrangement of

Substations 26 40

Connections for Substations 26 44

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viii CONTENTS

Power Transformation and Measurement

Continued Section Pagt
Measurement of Power on Polyphase Cir-
cuits 26 53

Instruments Used for Power Measurement 26 53

Indicating Wattmeters 26 54

Recording Wattmeters 26 54

Measurement of Power on Two-Phase Cir-
cuits 26 59

Measurement of Power on Three-Phase

Circuits 26 63

Installation of Recording Wattmeters . . 26 75
Testing and Adjusting Recording Watt-
meters .26 79

Reading Recording Wattmeters 26 82

Special Meters 26 85

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DESIGN OF ALTERNATING-
CURRENT APPARATUS

(PART 1)

ALTERNATORS

1. The design of alternators is in many respects similar
to that of multipolar continuous-current machines, many of
the parts being very similar. For example, the method of
calculating the field ampere-turns, and the design of the
field in general, is much the same in these two classes of
machines. A great many of the mechanical details are
also similar, and much of what has already been given as
applying to continuous-current machines applies also to
alternators.

3. Some of the calculations connected with the design
of alternators are, however, not so easily made as for direct -
current machines, and the production of a good design
depends largely on the skill and previous experience of the
designer. For example, there is a large variety of arma-
ture windings to select from, and the designer has to decide
which winding is best adapted for the work that the alter-
nator has to do. Such calculations as the estimation of
armature inductance, armature reaction, etc. are difficult to
make without having had previous experience with machines
of the same type as that being designed. The quantities
are, in general, easily determined after the machine has

§20

For notice of copyri^jht, see page immediately following the title page.

45—2

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2 DESIGN OF ALTERNATING § 20

been built, but their previous calculation is difficult. For
this reason the design of alternators is, on the whole, more
empirical than that of continuous-current machines. There
is also a greater choice as to the mechanical arrangement of
the different parts, since either the field or armature may be
the revolving member.

lilMITATION OF OUTPUT

3. The output of an alternator, like that of a direct-
current machine, may be limited by the heating of the arma-
ture. This heating is due to two causes, namely, the /' R
loss in the armature conductors, and the core loss due to the
hysteresis and eddy-current losses in the mass of iron con-
stituting the armature core. Both these losses appear in the
form of heat, and cause the armature as a whole to rise in
temperature. Since the maximum temperature at which an
armature can be run with safety is limited by the tempera-
ture to which the insulating material may be subjected con-
tinuously without injury, it follows that this heating effect
is an important factor, limiting the output of the machine.

4. The output may in some cases be limited by self-
induction and armature reaction. If the inductance of the
armature is very high, a considerable part of the E. M. F.
generated may be used to force the current through the
armature itself, thus reducing at the terminals of the
machine the E. M. F. available for use in the external cir-
cuit. In other words, if an alternator having an armature
with high self-inductance is run with a constant field excita-
tion, the voltage between the collector rings will fall off as
the load is applied. Most alternators have to be built under
a certain guarantee as to voltage regulation. By the volt-
age regulation is meant the percentage that the voltage rises
when the full load is thrown off an alternator. That is,
suppose an alternator, when carrying full load, generates
2,000 volts, and when the load is thrown off the voltage
rises to 2,100, the field excitation and speed remaining the
same. The increase is 100 volts, or 5 per cent, of the full-

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§20 CURRENT APPARATUS 3

load voltage, and the regulation would be 5 per cent ; the per-
centage always refers to full-load voltage, because full load
is taken as the normal operating condition of the machine.

6. In most of large slow-speed alternators of the revolv-
ing field type the ventilation is so good that the full-load
current can be delivered with a rise in temperature well
within the safe working limit. If, however, these machines
are not carefully designed they may not give the voltage
regulation required. The voltage may drop more than the
allowable amount when full load is applied because of the
armature reaction and self-induction. In such cases, there-
fore, the output that the machine can deliver without exceed-
ing the specified limit of voltage regulation may be limited
by the armature reaction and self-induction, and not by
heating. For certain classes of work close regulation is very
important, and in many cases the regulation becomes a more
important factor in the design, so far as limitation of output
is concerned, than heating.

As pointed out later, the regulation depends a great deal
on the character of the load that the machine carries. ' The
regulation might be very good on a non-inductive load and
so poor on an inductive load that the machine could not be
made to maintain its voltage even with the fields ej^cited to
the fullest extent. A statement of the regulation should
always include a statement of the character of the load for
which the regulation is given, i. e. whether non-inductive or
inductive, and, if the latter, the power factor.

6. In high-speed alternators, such as those driven by
belts or by steam turbines, the armature presents compar-
atively small surface for the dissipation of heat, and unless
special means are provided for ventilation, the heating effect
will be an important factor in determining the allowable
output. In direct-current machines, sparking at the com-
mutator often limits the output, but obviously this does
not apply to alternators, because no commutator is used,
except in some cases as an auxiliary part in connection with
the field-exciting circuit. However, while armature reaction

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4 DESIGN OF ALTERNATING §20

cannot cause sparking in an alternator it has a decided
influence on the voltage regulation, and its effects must
be carefully considered.

HEATING OP AXTERNATOU ARMATUBB8

7. The final temperature that an armature attains when
carrying its normal load depends not only on the actual
amount of energy wasted iji the armature, and that appears
in the form of heat, but also on the readiness with which
the armature can get rid of this heat to the surrounding
air. The armature will always keep on increasing in tem-
perature until it reaches a point where it radiates the heat
to the air as fast as it is generated. The rise in temper-
ature necessary to accomplish this will evidently depend
largely on the construction of the armature. A well-venti-
lated armature will get rid of more heat per degree rise
than a poorly ventilated one ; hence, every effort should be
made, in designing an armature, to arrange it so that the
air can circulate freely around the core and conductors.
This is best done by mounting the armature disks on an
open spider, and providing air ducts through the iron core,
which allow a circulation of air when the machine is run-
ning. By adopting this construction, makers have been
able to reduce the size of armature for a given output com-
pared with the size required for the same output when the
older style, with surface windings and unventilated core, was
used. The heat loss due to hysteresis and eddy currents in
the core is about the same, whether the machine is loaded
or not. Suppose an alternator to be run on open circuit
with its field fully excited. There will be no loss in the
armature- conductors, because the machine is furnishing no
current. The mass of iron in the core is, however, revolv-
ing through a magnetic field, and there will consequently
be a hysteresis loss in the iron, and eddy currents will be
set up in the armature disks. These will cause the arma-
ture to heat up until the rise in temperature is sufficient
to radiate these core losses. When the machine is loaded,

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§20 CURRENT APPARATUS 5

we have, in addition to the above, the heat loss in the con-
ductors due to the current that is now flowing. The result
is that the armature increases further in temperature until it
reaches a final temperature that allows the armature to get
rid of all the heat generated in it. If the armature is over-
loaded, the PR loss becomes excessive, and a point is soon
reached where it becomes unsafe to load the machine
further.

8. What was said regarding the safe heating limit of
the insulating materials used in the construction of con-
tinuous-current armatures applies also to armatures for
alternators. There is no good reason why an alternator
armature should be worked at a higher temperature than
that of a direct-current machine, although in many alterna-
tors, especially some of the older styles, the limit is much
higher. In modern machines, however, the rise of temper-
ature is very little, if any, higher than in continuous-current
machines of corresponding output and speed. The final
temperature when running fully loaded should not exceed 40°
to 50** C. above that of the surrounding air.

9. The total temperature that the armature attains
' when fully loaded depends on the temperature of the sur-
rounding air. It is not safe to count on less than 20° C. for
the average temperature of the surrounding air, because
the air in dynamo rooms in summer often goes far above
this. A fair rise in temperature may therefore be taken
as from 70° to 80° F., or from 40° to 50° C. These are
the ordinary values used in rating machines, and if an alter-
nator will deliver its full load continuously, with a rise
in temperature not exceeding the above, it should be per-
fectly safe, as far as danger from overheating goes. The
rise in temperature of the field coils is generally not quite
as high as that of the armature, but it must be remembered
that while the outside layers of the coils may be compara-
tively cool, the inner turns may be quite hot, and it is the
greatest temperature that any part of the coils attains that
must be taken into account.

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6 DESIGN OF ALTERNATING §20

REIiATION BETWEEN I' R liOSS AND OUTPUT

10. The /*R'\oss in an armature at full load usually
bears a certain ratio to the output of the machine. An
alternator with an excessive I* R loss in the armature con-
ductors would have a low efficiency. It is therefore impor-
tant that the armature be so designed that the heat loss in
the winding shall not exceed a certain proportionate amount
of the total output. This loss can be decreased by decreas-
ing the resistance of the armature winding. The resistance
can be decreased by either shortening the length of wire on

Curve shotting relation between artnature I^R loeuJt outjyut of alternator,

PlO. 1

the armature or by increasing its cross-section. A certain
length of active conductor is necessary for the generation
of the E. M. F. ; hence, to keep down the /^R loss, we must
use an armature conductor of large cross-section. The
size of conductor, if increased too much, calls for a large
armature for its accommodation, and the machine is thus
rendered bulky and expensive. All that can be done, there-
fore, is to design the armature winding so that the heat loss
will be as small as is consistent with economy of construc-
tion. Older types of alternators had a large armature

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§20 CURRENT APPARATUS 7

/* R loss, but the curve drawn in Fig. 1 may be taken as
giving the average loss for ordinary alternators. The
abscissas of this curve give the output in kilowatts, and the
ordinates, the PR loss in per cent, of the output. It will
be understood that the loss in individual machines might
vary somewhat from the values shown, but the curve shows
the average relation for machines where the /* R armature
loss is not excessive. It will be noticed that this loss is a
much larger percentage for small machines than for large
ones. For machines over 100 K. W., the percentage loss
does not decrease much with increased output.

CORE liOSSES

11. The core losses have already been mentioned as one
of the causes producing heat in the armature. These losses
are present also in continuous-current armatures, but their
effects are usually much less than in alternators. In some
alternators the core losses are nearly if not quite as great as
the PR loss, and consequently the no-load rise in tem-
perature may be considerable.

HYSTERESIS LOSS

12. The nature of this loss has already been explained
in connection with the design of direct-current machines
and the method of calculating it pointed out, so that it will
not be necessary to dwell further on it here. The curves
shown in Fig. 2 will be found useful for calculating the
hysteresis loss in alternating-current apparatus. Curve A
shows the relation between the maximum magnetic density
and the watts lost per cubic inch per 100 cycles for a good
quality of soft transformer iron. Curve B shows the loss for
ordinary armature iron of good quality. In order to obtain
the total hysteresis loss for a given mass of iron, multiply
the value given by the curve corresponding to the maximum
density at which the iron is worked, by the volume in cubic
inches and the frequency and divide the result by 100.

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8 DESIGN OF ALTERNATING §20

Example. — The armature core of an alternator having 12 poles and
running at a speed of 600 revolutions per minute is worked at a maxi-
mum magnetic density of 20,000 lines per square inch. If the volume
of the core is 2,t)00 cubic inches, how many watts will be wasted in
hysteresis ? •

Magnetic d&mtity B {lines per bq. inch)
FlO. 2

Solution. — If the machine runs at 600 rev. per min. and has 12 poles,
the frequency of the magnetic cycles in the armature core must be
V X Vo"' ^r 60 cycles per second.

By referring to curve //, Fig. 2, we find the loss per cubic inch
per 100 cycles corresponding to a density of 20,000 to be about .22 watt.
Hence, the total loss will be

... .22x2.000x60 __. ^^ .
Wu = ^.^. = 264 watts. Ans.

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§20 CURRENT APPARATUS 9

13« The hysteresis loss, other things being equal,
increases directly with the frequency. It is on this account
that this loss is usually greater in alternator armatures than
in those used for direct-current machines, because the fre-
quency of the former is usually much higher than that of
the latter. Special care should therefore be taken in the
selection of core iron for all kinds of alternating-current
apparatus. It will also be noticed that the hysteresis loss,
being proportional to the 1.6th power of the magnetic
density, will increase quite rapidly as the density is increased.
It follows, therefore, that the core densities used should be
low, otherwise the hysteresis loss may become excessive. It
is usual to employ lower core densities in alternating-cur-
rent machines than in continuous-current machines, because
the frequency is usually fixed by the conditions under which
the machine has to work, and a low density is therefore
necessary to keep down the hysteresis loss.

BDDY-CURREXT IX>SS

14. The other core loss mentioned above, namely that
due to eddy currents, is not usually very large, provided
proper care is taken in building up the armature core.
This loss is due to local currents circulating in the armature
disks, and the eddy-current loss is really an /' R loss caused
by the resistance offered to these currents by the iron con-
stituting the core. If the core is thoroughly laminated, the
paths in which these currents flow are so split up that the
currents are confined to the individual armature disks.
This keeps down the volume of the eddy currents, and if
the disks are well insulated and made of thin iron, the eddy-
current loss may be made very small. Anything that
makes electrical connection between the disks may largely
increase this loss. For example, filing out the slots, or
burring over the disks, or passing uninsulated clamping
bolts through the core may result in an increased loss. It

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10 DESIGN OF ALTERNATING §20

is well, therefore, to avoid filing or milling the slots unless
it is absolutely necessary to render them smooth enough to
receive the insulating troughs and armature conductors.
The eddy-current loss is proportional to the square of the
frequency, other things being equal; hence it is usually
greater in alternators than in direct-current machines. If
proper precautions are taken in building up the core, the
eddy-current loss should be small compared with the /* R
and hysteresis losses. It is difficult to calculate this loss
beforehand, on account of the large variations caused in it
by defects in the insulation of the core disks from each
other.

RADIATING SURFACE OF ARMATURE

16. The armature has to present sufficient radiating
surface to get rid of the heat dissipated without a rise in
temperature exceeding, say, 40° or 50° C. This means that
the size of the armature will, for a given output and given
amount of loss, depend on the ease with which it can radiate
the heat. The number of watts that an armature can
radiate per square inch of surface per degree rise in tem-
perature varies greatly with the style and construction of
the armature and the peripheral speed at which the arma-
ture is run, so that it is not possible to give any values for
this radiation constant that will be applicable to all styles
of armatures. A well- ventilated iron-clad alternator arma-
ture should be able to radiate from .04 to .06 watt per
square inch of cylindrical surface (circumference of iron
core X length parallel to shaft) per degree rise. These
values are for machines running at peripheral speeds of
from 4,000 to 5,000 feet per minute; if the peripheral speed
were higher, the watts radiated per square inch per degree
rise would be correspondingly increased. This means, then,
assuming 40° C. to be the allowable rise, that a well-
ventilated armature of the above type should be capable of
radiating from l.G to 2.8 watts per square inch of cylindrical

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§20 CURRENT APPARATUS 11

surface. In well-designed alternators, the sum of the hys-
teresis and eddy-current losses will not, as a rule, be greater
than the /*AMoss, so that we will, in general, be safe in
assuming that an allowance of from .8 to 1.4 watt /' R loss
for each square inch of surface will give an armature of
sufficient radiating surface to keep the total rise in tempera-
ture due to all the losses from exceeding 40° C. This will
give a preliminary value for the surface of the armature on
which to base subsequent calculations, bearing in mind that
the dimensions so obtained are not necessarily final, and
may be modified as the design is worked out further, pro-
vided always that the armature is made of such dimensions
that it will be able to get rid of the heat generated.
Machines have been built in which the surface per watt is
less than that given above, but it will usually be found that
such machines run very hot when fully loaded unless their
peripheral speed is very high or their ventilation exception-
ally good. Alternator armatures of the iron-clad type can
usually be constructed so as to secure good ventilation,
especially if they are of fairly large diameter, so there
should be no difficulty in radiating the amount of heat just
given. The watts per square inch as given are referred to
the outside cylindrical surface; of course, the ends of the
core, and to a certain extent the inside also, help to radiate
the heat, but it is more convenient for purposes of calcula-
tion to refer the watts radiated per square inch to the out-
side core surface rather than to the surface of the armature
as a whole.

ARMATURE REACTION

16. Armature reaction, in connection with alternators,
has already been mentioned in a general way, and it now
remains to be seen just how it affects the action of a machine
when loaded. The matter of armature reaction plays an
important part in the design of continuous-current machines,
as has already been shown in the section on the design of

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DESIGN OP ALTERNATING

such dynamos. If the armature of a continuous-current
machine is capable of overpowering the field, bad sparking
will result at the commutator. This, however, cannot occur
in the case of an alternator, and the only bad effect that the
reaction can have is to cause a weakening and distortion of
the field, with a consequent reduction of the voltage gener-
ated in the armature.

PlO. 8

17. Let N^ Fig. 3, represent one of the north poles of
an alternator, surrounded by its magnetizing coil a. The

lines of force will flow into the
armature from the pole piece,
as indicated by the lines and
arrowheads. We will consider
the instant when the coil c c'
has^ its opening directly under
the pole, or when the center
of the tooth b is opposite the
center of the pole piece. If
there is no self-induction pres-
ent, the current flowing through
the armature will be in phase
with the E. M. F. generated ; consequently, at the position
shown in the figure, the current in the coil will be zero,
because the coil is cutting no lines of force, and the E. M. F.
generated is consequently zero. It follows, then, that
under this particular set of conditions
the armature coil has no disturbing
effect on the lines of force set up by the
field. The direction of rotation is indi-
cated by the arrow, and a moment later
the bundle of conductors in the slot c is
under the center of the pole, as shown in
Fig. 4. The current in the conductors
will now be at its maximum value, be-
cause the E. M. F. generated is at its
maximum. The current will be flowing down through the
plane of the paper, and the bundle of conductors lying in

PIO. 4

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CURRENT APPARATUS

PIO. 6

the slot will tend to set up lines of force around themselves,
as shown by the dotted lines, and in the direction shown by
the arrowheads. It will be noticed
that this field set up by the conductors
tends to strengthen the right-hand side
of the pole and weaken the left-hand
side by a like amount. The resultant
effect is therefore to crowd the field
forwards in the direction of rotation,
making it denser at the right-hand side,
as shown in Fig. 5. It is therefore
seen that in this respect the effect of
armature reaction is similar to the
effect observed in direct-current machines; but in an alter-
nator with coils, as shown in the figures, the effect on
the field is not steady, but varies as the teeth move past
the poles. The student should note that in this case the
armature and load are assumed to have no self-induction,
and also that the armature reaction tends only to change the
distribution of the field and not to weaken it.

18. Armatures always have more or less self-induction,
especially if they are provided with heavily wound coils sunk

in slots. The effect of this
self-induction is, of course,
to cause the current in the
armature to lag behind the
E. M. F. It is necessary,
then, to see how this lagging
of the current affects the
reaction of the armature on
the field. In this case the
current in the coil does not
die out at the same instant
as the E. M. F., but persists
in flowing after the E. M. F. has become zero. The cur-
rent, instead of being zero when the tooth is under the pole,
will then be flowing as shown in Fig. 6 ; that is, the current

PlO. 0

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14 DESIGN OF ALTERNATING §20

in the conductors in slot c persists in flowing, as 'shown in
Fig. 5, after the conductors have moved out from under the
pole piece. This current flowing in the armature coil will
set up lines of force through the coil in the direction shown
by the dotted arrows, i. e., directly opposed to the original
field. The armature reaction, therefore, not only tends to
distort the field, but also tends to weaken it when there is
a lagging of the armature current due to self-induction in
the armature or external load. This reaction of the arma-
ture on the field would of course cause a falling off in the
voltage of the machine if the field magnets were not
strengthened to counterbalance its effects. It is instructive
to note here that if it were practicable to have a condenser
in connection with the armature, the current could be made
to lead the E. M. F., and the armature reaction would then
tend to magnetize the field instead of demagnetize it.

19. It is seen from the above that in alternator arma-
tures in which there is an appreciable amount of self-induc-
tion present, we have two effects similar to those produced
by the cross ampere-turns and back ampere-turns of a
continuous-current armature, the former tending to distort
the field, and the latter acting directly against it and tend-
ing to weaken it. The bad effects of this reaction can be
reduced, as in the case of direct-current machines, by length-
ening the air gap. The actual amount of distortion or
demagnetization is not easily calculated, as it evidently
changes with the changes in the current, and also depends
on the armature inductance, which is itself difficult to esti-
mate without data from machines of the same type. The
distribution of the field can be determined after the machine
has once been built, and unless the air gap is very short,
the distortion is not sufficient to badly affect the working
of the machine.

20. One effect of armature reaction is sometimes taken
advantage of in designing armature windings, namely, the
crowding together of the lines to one side of the pole piece.

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§20 CURRENT APPARATUS 15

This practically makes the effective width of the pole face
less and allows the use of coils on the armature with an
opening somewhat less than the width of the pole face,
without danger of the E. M. F.'s in the different turns of
the coil opposing each other.

ARMATURE SELF-INDtJCTIOX

21. It has just been shown that self-induction is indi-
rectly responsible for the demagnetization of the field,
which in turn produces a falling off in voltage. Self-
induction also calls for a considerable E. M. F. to force
the current through the armature, and this causes a still
further diminution in the E. M. F. obtained at the ter-
minals. This drop in voltage has already been explained in
the section on Alternators, A machine with high armature
self-induction will not maintain a constant terminal pres-
sure unless the field is strengthened as the load is applied,
and such machines therefore require heavily compounded
fields.

22. In general, armatures wound with a few heavy coils
bedded in slots have a high self-induction, because the coils
are able to set up a large number of lines around themselves
when a current flows through the armature. Machines with
this style of armature winding usually give an E. M. F.
curve that is more or less peaked and irregular. Such
windings are easily applied to the armature, and being of
very simple construction, they necessitate very few crossings
of the coils at the ends where the coils project from the
slots. They are, therefore, easy to insulate for high volt-
ages, and are extensively used on alternators for operating
incandescent lights.

23. The inductance depends on the way in which the
coils are arranged in the slots. Fig. 7 (a) shows a cross-
section of a slot containing a heavy coil of 40 turns. When
current is passed through the coil, a magnetic field is set up

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16 DESIGN OF ALTERNATING §20

that encircles the coil as indicated by the dotted lines. The
self-induced E. M. F. will depend on the strength of this
field and on the number of turns with which the field is
linked. The strength of field depends on the current, the
number of turns, and the reluctance of the magnetic path
surrounding the turns. If the reluctance is a constant
quantity, it is evident that the self-induced E. M. F. for a
given current will increase as the square of the number of

(m)

/^M^/»/K'Z:

Fig. 7

turns per coil or conductors per slot. Such being the case,
the inductance could be decreased by splitting up the single
coil into two or more coils placed in separate slots, thus
reducing the number of conductors per slot. For exam-
ple, suppose an armature has 6 coils of 40 turns each, and
that the inductance of each coil is .02 henry. The coils
are supposed to be connected in series, so that the total
inductance of the armature will be G X .02 = .12 henry.

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§20 CURRENT APPARATUS 17

Suppose, now, the winding is split up into 12 coils of 20 turns
each, the shape and arrangement of the coils being kept the
same. We will then have the same total number of turns
as before, but will have half as many turns per coil or half
as many conductors per slot. The inductance of each coil
will therefore be one-fourth of what it was before, because
the inductance will decrease as the square of the number of
turns per coil. The inductance per coil will then be ^ X 02
= .005 henry, and the total inductance will be .005 X 12
= .06 henry, or one-half of what it was in the former case.
In order, then, to decrease the inductance of an armature,
the number of turns per coil must be decreased, or, what
amounts to the same thing, the number of conductors per
slot must be decreased.

In the preceding example, it has been assumed that the
reluctance of the path around the coil is the same for the
heavy coil as for the light coil. This, however, is not the case
in practice, and the reduction of inductance by subdividing
the winding is not as great as the theoretical example
just given would indicate. In Fig. 7 (^), it will be noticed
that the greater part of the reluctance of the magnetic
path occurs at the air gaps around the top of the slots, as
indicated at a b. With a wide shallow slot, the reluctance
of the path c d between the sides of the slot is also larger.
When the coil is split up, it is necessary to use narrower
slots and teeth, as shown at (b), so that the air gap ab \^
made much shorter. Also, the slots being deep and narrow
compared with (^), the reluctance between the sides of the
slot itself is less. The result is that the decrease in the
number of conductors per slot may be offset to a consider-
able extent by the decreased reluctance, so that the product
of the flux times turns may not be reduced to nearly so
great an extent as the decrease in the turns per coil would
lead one to expect. With the narrower slots in (^), the
higher tooth density tends to keep up the reluctance of the
magnetic path, but saturated teefh are not used as much in
alternators as in direct-current machines, and the tendency
of making the slots narrower and deeper is, on the whole,

45—3

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18 DESIGN OP ALTERNATING §20

to reduce the reluctance of the path for the magnetic flux
that is responsible for the setting up of the induced E. M. F.
While, therefore, the splitting up of the winding does not
reduce the inductance in proportion to the square of the
number of turns per coil, yet it does reduce it considerably,
and for machines where iow armature inductance and close
voltage regulation are desired, the winding is usually split
up in the manner described. This subdivision of the wind-
ing will be described more fully later.

JS4. Calculation of Armature Inductance. — Since the
inductance of the armature coils depends on the reluctance
of the magnetic path around the coils, it is evident that it
' will be influenced not only by the size and shape of the
slots, but also by the position of the armature with regard
to the field, and also by the length of the air gap between
armature and field. For example, in Fig. 7 {a), when the
bundle of conductors is under the poles, as shown, the
inductance is a maximum because the iron pole face helps
to carry the flux around the conductors. If the air gap
were very short, it is evident that the reluctance of the
path for the induced flux would be much less with the slot
under the poles than when between the poles, because in
the latter case the path between the tops of the teeth would
be wholly through air. It is evident that with a long air
gap there would be little difference in the inductance under
the poles and between the poles. The inductance is there-
fore not constant, but varies with the position of the slots
with regard to the pole pieces. It is also evident that the
number of lines set up through a coil will be proportional to
the length of the laminated core, i. e., the length parallel to
the shaft, so that for an equal number of turns, short arma-
atures have a lower inductance than long ones.

25. On account of the number of variable qua'ntities
that enter into the calculation of the inductance, it is not
possible to lay down any rule that will apply to all sizes of
slot, air gap, length of core, etc. Inductance calculations

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§20 CURRENT APPARATUS 19

are based on data obtained from tests of machines of similar
type to the one being designed. Parshall * gives a number
of tests made to determine the inductance of various arma-
tures and shows that the field set up around a coil varies
from 13 to 140 or 150 lines per ampere-turn per inch length
of armature core. The latter high values are for armatures
with a very short air gap and with the conductors under
the poles in the position of maximum inductance. For
fairly wide slots, and with the conductors in the position of
minimum inductance between the poles, the value is
from 15 to 20 lines per ampere-turn per inch length of core.
For example, suppose an armature coil had 40 turns and
that we take 20 lines per ampere-turn per inch length of
core as a fair value for the field set up around the coil.
Also, suppose that the armature core is 8 inches long. The
flux through the coil will then be 20 X 8 X 40 = 6,400 lines
for a current of 1 ampere. We have

<PX T _ -

where (P is the flux corresponding to a current of 1 ampere,
T the number of turns, and L the inductance in henrys.
Then, in this case,

,. 6,400 X 40 ^^^^^ ,

L = ' 3 — = .00256 henry

The probable value of the flux can usually be calculated
from data obtained from tests on similar machines, and data
of this kind is absolutely necessary if accurate estimates of
inductance are to be made. The preceding example will,
however, give the student an idea as to the elements on
which the value of the inductance depends. If the induct-
ance L is known, the armature reactance is easily obtained
from the expression ^-nn L, where n is the frequency. The
voltage necessary to overcome the reactance is 2nnLI^
where / is the current in the armature.

**• Electric Generators," Parshall and Hobart

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20 DESIGN OP ALTERNATING §20

Alternators provided with armatures of low inductance
give a much better E. M. F. regulation than those having
high inductance, because the reaction on the field is
not only less, but much less of the E. M. F. generated is
used up in driving the current through the armature. In
other words, such machines, if provided with a constant
field excitation, will show only a moderate falling off in
terminal voltage from no load to full load. On this account,
it is quite common to find such machines built without any
compound or series-winding on the fields, all the regulation
necessary being accomplished by varying the current sup-
plied to the field coils by the exciter. Such alternators give
a smooth E. M. F. curve that approximates closely to the
sine form, and alternators of this type are being used exten-
sively for power-transmission purposes.

26. An excessive amount of armature inductance, and
consequent damagnetizing armature reaction, has been used
to make alternators regulate for constant current. In such
machines the armature inductance is made very high, and a
small air gap is used between the armature and field. If
the current delivered by such- a machine tends to increase
by virtue of a lowering of the external resistance, the arma-
ture reaction on the field increases and the field is weak-
ened. This cuts down the voltage generated, so that the
voltage adjusts itself to changes in the load, and the cur-
rent remains constant.

PERIPHERAIi SPEED OF AliTERNATOB
AR^IATURES

37. Alternators have been built to run at peripheral
speeds much higher than those used for continuous-current
machines. This was the case in many of the older types of
lighting machines running at a high frequency. Since the
frequencies employed were high, the revolutions per minute
of the armature also had to be high in order to avoid using
a very large number of poles. This high speed of rotation
usually resulted in high peripheral speeds also, because the

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§20 CURRENT APPARATUS 21

armature could not be made very small in diameter. Such
machines often ran at peripheral speeds as high as 7,000
or 8,000 feet per minute. Modern revolving-field machines
for direct connection to waterwheels often run 7,000 or
8,000 feet per minute, and steam turbine alternators from
12,000 to 15,000.

5S8. The frequency of a great many modern machines
is lower than that formerly used, 60 or 26 cycles per second
being standard values. The lowering of the frequency
was accompanied by a lowering of the peripheral speed,
and the peripheral speeds of revolving armature alternators
compare favorably with those of multipolar direct-current
machines of the same output. Peripheral speeds for belt-
driven 60-cycle alternators may be taken from about 3,600
to 6,600 feet per minute. The peripheral speed of some of
the larger direct-connected alternators may be even lower
than this, just as the peripheral speed of multipolar direct-
current generators is usually lower than that of belt-driven
machines. Alternators of the inductor or revolving field
construction can be run at higher peripheral speeds than
those with a revolving armature on account of the mechani-
cal construction of the revolving field or inductor being
more substantial than that of a revolving armature.

ARMATURE WINDINGS

29« The foregoing articles have dealt with different
subjects relating to the behavior of armatures. We will
now take up those subjects that deal more particularly
with their design. Some of the most important points in
the design of an armature are the selection of the type of
winding to be used for a given case, the method of connect-
ing it up, and the means used for applying the winding to
the armature. Alternator windings have already been dealt
with to some extent in the section on Alternators^ but the
following articles are intended to bring out some points of
difference between concentrated and distributed windings

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22 DESIGN OF ALTERNATING §20

that are necessary for the designing of armatures for alter-
nators and fields for induction motors.

30. Alternator windings may be divided into two gen-
eral classes, namely: (a) uni-coil or concentrated wind-
ings; {d) multi-coil or distributed windings. These may
further be subdivided into (1) uni-coil single-phase wind-
ings; (2) multi-coil single-phase windings; (3) uni-coil poly-
phase windings; (4) multi-coil polyphase windings.

The uni-coil windings for single-phase, two-phase, and
three-phase machines have been treated in the section on
Alternators, We will presently examine single-phase multi-
coil, or distributed windings, to see how the spreading out of
the winding affects the voltage generated by the armature.

SINGL.B-PHASE CONCENTRATED WINDING

31. A single-phase concentrated winding has only one
slot or bunch of conductors under each pole ; consequently,
the conductors are practically all active at the same instant,
and the maximum E. M. F. is obtained with a given length
of active armature conductor. This E. M. F. is given by
the formula

^ 4.44 ^ Tn '

^ = 10- -

where T = number of turns connected in series on the
armature ;
^ = total magnetic flux from one pole;
n = frequency;

i? = E. M. F. generated in armature, or E. M. F.
obtained between the collector rings at no-
load.
Such windings have therefore the advantage of giving a
high E. M. F. for a given length of conductor, but they
have the disadvantage that they give rise to high armature
self-induction and consequent falling off in terminal voltage
when the machine is loaded. Also, the heating of the coils
is likely to be greater than if they were spread out.

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CURRENT APPARATUS

SINGLE-PHASB DISTRIBUTED WINDINGS

3!8« It has been shown that the self-induction can be
reduced by splitting up the coils and distributing them over
the armature. Such distribution is, however, always accom-
panied by a lowering of the E. M. F. generated, even
though the total number of turns be kept the same. Sup-
pose, for example, we have a single-phase armature with
T turns, connected in series and arranged with only one
slot or bunch of conductors under each pole. The E. M. F.
generated will then be

i? =

4.44 <P Tn
10-

Suppose, now, we spread the winding out so that there
will be two sets of conductors or two slots for each pole, and

Pio. 8

distribute these slots equally around the armature. We will
put half as many conductors as before in each slot, so that

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DESIGN OF ALTERNATING

§20

the total number of conductors and turns will remain the
same as before. This will give us a winding similar to that
shown in Fig. 8. This shows an eight-pole single-phase
winding with two slots per pole piece. By examining the
figure, it is evident that with such an arrangement the con-
ductors in slot b are, at the in-
stant when they are directly
between the poles, generating
zero E. M. F., while those
in a are generating the maxi-
mum E. M. F. The E. M. F.
that will be obtained between
the collector rings will be the
sum of the two, as shown in
Fig. 9. Oa represents the
E. M. F. generated in one set
of conductors, while O b repre-
sents the E. M. F. generated in the other. These two
E. M. F. 's will be equal, and will be given by the expression

- £44±Tn^j

PiO. 0

^^LU^^^ (1)

since there are ^ the total turns T active in each set.
resultant E. M.¥,Oc will therefore be

The

^ 4.44 ^Tn , ^ 4.44 ^ Tn „^„ ,^.

E = —. X i X V2 = j^, X .707 (2)

10"

That is, the E, M, F. that is obtained at no load from a
twO'Coil single-phase winding is ,707 times that which would
have been obtained with the same total number ofturtis grouped
into a uni'Coil winding. By spreading out the winding in
this way, the no-load voltage has, for the same number of
active conductors, been reduced about 30 per cent. ; the
inductance of the armature has, however, been reduced
considerably ; so that, although we may not get an armature
that will give as high a voltage at no load, it may give as

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CURRENT APPARATUS

high a terminal voltage when loaded, and a machine pro-
vided with such a winding would hold its voltage more
nearly constant throughout its range of load.

33. The subdivision of the winding might be carried
still further, and three slots for each pole piece used. The
E. M. F.*s in the three sets of conductors would then be
related as shown in Fig. 10. Each of the groups would

{

PlO. 10

consist of — turns, and the three E. M. F.*s O a^ O b^ and Oc

would be displaced 60"^ from each other, instead of 90°, as
shown in Fig. 9, because there are three groups of conduc-
tors per pole, and the distance from center to center of the
pole pieces corresponds to 180°. The E. M. F. generated in
each set will be

and the resultant E. M. F. O d. Fig. 10, will be

(3)

^ = iifAZ:^X| = i:MAZ:^X.667

10"

(4)

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26 DESIGN OF ALTERNATING §20

The effect of spreading out the coils into a three-coil
winding is, therefore, to reduce the no-load terminal E. M. F.
still further, and at the same time to reduce the self-induc-
tion. It will be noticed that the difference in the voltages
given by a two-coil and by a three-coil winding is not nearly
so great as that between the voltages of the two-coil and
single-coil windings. If the winding is spread out still more,
the E. M. F. generated is reduced by very little, and if the
subdivision is carried out so that the winding becomes uni-
formly distributed over the whole surface of the armature,
the formula becomes

r^ 4.44 ^ Tn ^^^ ,-.

£= —, X.636 (5)

34 'S^^ more the winding is spread out, the greater the
number of c*^<^ssings of the coils at the ends of the armature,
making such wi^^^^^^s difficult to insulate for high voltages.
Such windings, ^ ^''efore, have the disadvantage of being
more expensive?^ -struct and insulate, in addition to

giving a lower l£ >, ' ^^ ^^^d for a given length of

active conductor. L r ' : '^^ advantage of giving better

regulation or small aa ,c t;.^ --r ''''" loaded, and also

give a smooth E. M. F. curve. v^,^;.^heating is more
uniformly distributed than when a concentrated T'^iding is
used. For single-phase armatures in general, we may tn^^.
write the E. M. F. equation as follows:

£ = il^X>t (6)

where T = total number of turns connected in series on the
armature ;
^ = total flux from one pole ;
n = frequency;

k = constant depending on the style of winding
used.
For a single-coil or concentrated winding, k = 1; for a
two-coil winding, y^ = . 707 ; for a three-coil winding, * = . 667 ;
for a uniformly distributed winding, k = .636.

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20 CURRENT APPARATUS 27

POLYPHASE ARMATURE WINDINGS

35. Concentrated, or uni-coil, polyphase windings have
already been described in the section on Alternators. The
two- and three-phase windings there described consist of one
group of conductors, or one slot for each pole and each
phase. Polyphase windings can, however, be distributed in
a manner similar to that just given for single-phase wind-
ings, and such distributed windings are in common use for
induction motors, polyphase alternators, and polyphase syn-
chronous motors. The distribution of such windings is
accompanied by a lowering of the terminal E. M. F., as in
the case of single-phase windings, though this decrease in
the E. M. F. is not nearly so great. Suppose, for example,
we have a three-phase winding with two groups of conduct-
ors per pole per phase. We will have then six groups of
conductors for each pole, and as the distance from center to
center of poles is equivalent to 180°, the E. M. F.'s in the two

180°
groups of each phase will differ in phase by - ^ -, or 30°.

Let the total number of turns per phase be T. Then, the

number of turns in each of the two sets constituting each

T
phase will be — , and the E. M. F. generated in each of the
2

sets will be

^ c- 4.44 4>r« ,

£.^E.= 10- >< ^

These two E M. F's will be related as shown in Fig. 11,
and the resultant E. M. F. will be

r^ 4.44 ^ Tn , ^ ^^o
E = —, X i X 2 cos 15°

= ^^^X.965 (7)

Hence, the voltage generated per phase by a two-coil three-
phase winding is . 965 times that zuhich ivonld be generated by
a single-coil zvindiftg. In other words, the splitting up of
the winding has resulted in a voltage reduction of but

Digitized by VjOOQIC

28 DESIGN OP ALTERNATING §20

3J per cent. If a three-coil winding were used, the E. M. F.
would be reduced still further, and if a uniformly distrib-
uted winding covering the whole surface of the armature
were employed, the constant would become .95. If a uni-
formly distributed winding is used on a two-phase machine,
the value of the constant becomes .90. For polyphase

PIO. 11

windings we may then summarize the following: The
E. M. F. generated per phase in a polyphase armature is
given by the expression

j^ 4.44 ^Tn , ,Q.

E = Yo" ^ ^ ^^^

where T = number of turns connected in stries per phase ;
^ = flux from one pole ;
n = frequency;

k = constant depending on the arrangement of the
winding.

For a winding with one group of conductors per pole per
phase, k = 1; for a two-phase winding uniformly distrib-
uted, ^= .90; for a three-phase winding uniformly dis-
tributed, k = .95; for a three-phase winding with two
groups of conductors per pole per phase, k = .905.

The student will notice particularly that formula 8 gives
the voltage per phase, not the voltage between the collector
rings or terminals of the machine. This latter voltage will
evidently depend on the method adopted for connecting the
different phases together.

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\QiiS

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20 CURRENT APPARATUS ' 29

ARRANGEMENT OF WINDINGS

36. The method of arranging these distributed windings
will be understood by referring to Figs. 12 and 13. Fig. 12
shows a six-pole two-phase coil-wound armature with two
slots per pole per phase. The coils are shown by the heaVy
outlines, the winding being in two layers, so that there are
as many coils as slots. Only one phase is drawn in complete,
so as not to confuse the drawing. Take the coil A. One
side e of this coil lies in the top of a slot, and the other
side / lies in the bottom of the corresponding slot under the
next pole. The light lines a, a' represent the terminals of
the coil A, and the light connections show the connections
between the coils constituting one phase. Starting from
collector ring i, we pass from a around coil A and come
to a'; a' is joined to ^, so that the current passes around
coil B in agreement with the arrows; the terminal t' is then
connected to c\ so as to pass through coil C in the direction
of the arrows. This process is repeated until the twelve
coils constituting the phase are all connected in series and
the remaining terminal / is brought to collector ring 2,
The other phase, of which the active conductors are indi-
cated by the light lines, is connected up in exactly the
same way and its terminals brought to the collector rings 3
and 4' This gives a completed two-phase winding that
consists of two coils for each pole and each phase, all the
coils in each phase being connected in series and each phase
connected to its pair of collector rings.

37, Fig. 13 represents a three-phase bar-wound arma-
ture with two slots for each pole and each phase. The
armature is wound for eight poles, so that there are 32 bars
or conductors connected up in series in each phase. One
phase is shown connected up, the conductors belonging to the
other two phases being indicated by the dotted and dot-and-
dash lines. Starting from the collector ring r„ we connect
to the bottom conductor in slot /; from there we pass to
the corresponding slot under the next pole, that is, slot 7,

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30 ' DESIGN OF ALTERNATING §20

and connect to the top conductor in that slot. In this way
we pass twice around the armature, connecting up the bars
in accordance with the arrows, coming finally to the point n.
From n a connection is made to m^ and from m we pass
twice around the armature again in the opposite direction,
and come finally to the point j, which is connected to the
common junction >& if a Y winding is employed. This con-
nects all the conductors belonging to this phase in series.
The bars constituting the other two phases are connected
in a similar way, and the three phases connected up in
the Y or A combination, according to the rules that have
been given in the section on Alternators. A three-phase
alternator X provided with a winding like that shown in
Fig. 13 would be suitable for a machine designed to deliver
a large current output at a low voltage. In such a case,
the number of armature conductors required would be com-
paratively small, and bars could be used to advantage. A
similar scheme of connection could be used for a coil-wound
armature, except that each element of the winding would
consist of a number of convolutions instead of the single
turn, as shown in Fig. 13.

38. By referring to Figs. 12 and 13, it will be noticed
that in such two-layer windings the top conductors are
always connected across the front and back of the arma-
ture to bottom conductors ; that is, a conductor in the top
of one slot is not connected to the top conductor in the
corresponding slot under the next pole. This is done to
make the arrangement of the end connections such that
they do not interfere with each other as already explained
in connection with direct-current dynamos. The two-layer
type of winding is on this account extensively used, and its
application will be taken up further in connection with
induction-motor design.

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CURRENT APPARATUS 31

CONSTRUCTION OF ARMATURES

39. On the whole, the mechanical construction of alter-
nator armatures is very similar to that employed for arma-
tures for multipolar direct-current machines. There are
differences in the electrical features, arising from the differ-
ent type of winding usually employed and the absence of
commutator connections. The construction of many of the
armatures is simpler than that necessary for continuous-
current machines, on account of the smaller number of coils
used in making up the armature winding.

ARMATURE DISKS

40. Most of the armature disks used are adapted for
armatures of the drum type. Such disks or disk segments
are stamped from well-annealed mild steel. It is essential
that whatever material is used, the hysteresis factor should
be low, especially if the armature is to be run at a high fre-
quency. It is almost the universal practice at present to
use toothed cores, although smooth-core armatures were
quite common in some of the older types of alternators.
Core iron should be from .014 in. to .018 in., or from 14 mils
to 18 mils, thick. Iron thicker than this is frequently used
in direct-current machines, but it is not safe to use iron
much thicker in alternator-armature cores on account of the
danger of increasing the eddy-current loss. Some makers
depend on the oxide on the disks for the insulation to pre-
vent eddy currents, while other makers give the disks a
coat of japan before they are assembled to form the core.

41. The variety of disks used for alternator armatures
is large. Some are designed for stationary armatures of
large diameter, while others are for rotating armatures of
comparatively small diameter. The different styles of slots
used are also numerous. Fig. 14 represents a common style

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32 DESIGN OF ALTERNATING §20

of disk used for lighting alternators. This disk is provided
with as many teeth and slots as there are poles on the alter-
nator. Each tooth is provided with the projections ^, a^

which hold the coils in
place and obviate the ne-
cessity of band wires. A
keyway k is provided by
which the disks are keyed
to the spider supporting
them. It is well to notice,

in passing, that core disks
for alternators are usually
quite shallow, the depth of
iron d under the slots being
small compared with that
usually found in direct-cur-
^o. 14 rent armatures, making the

disks appear more like rings. This is accounted for by the
fact that in an alternator the total flux that the armature
conductors cut in one revolution is divided up among a
large number of poles; consequently, the flux from any one
pole is comparatively small. The flux through the core
under the teeth is one-half
the flux from the pole
piece; the cross-section of
iron necessary to carry it is
therefore small, and a large
depth of core is unneces-
sary to obtain the required
cross-section.

43. Fig. 15 shows an-
other style of disk and slot
in common use. This disk
is provided with 16 slots,
and would be suitable for ^'°- ^^

an eight-pole two-phase winding. The same style of disk
with 24 slots would answer for the three-phase winding.

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§20 CURRENT APPARATUS 33

The disk shown in Fig. 15 is provided with slots that have
dovetailed grooves near the circumference. After the coil
is placed in position, a wooden wedge is fitted into these
grooves, thus holding the coil firmly in place and doing
away with the necessity of band wires.

43. When the armature is wound with bars, straight
slots are frequently used. Fig. 16 shows such a disk pro-
vided with 48 equally spaced
slots. A disk of this kind
would be suitable for an
armature core for the wind-
ing shown in Fig. 13. It
would be necessary in this
case to use band wires to I
hold the conductors down
in place, giving a construc-
tion very similar to that
commonly employed for
direct-current armatures.

44. Stationary arma- fig. le »

tures for large machines are placed externally to the revolv-
ing field, and the coils are placed in slots around the inner
periphery. Since such armature cores are generally of large
diameter, the armature disks have to be punched out in
sections, as shown at c in Fig. 17. These sections are pro-
vided with dovetail projections b that fit into slots in the

Pig. 17

supporting iron framework A. As the core is built up, the
joints between the different segments are staggered, or the

45—4

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84 DESIGN OP ALTERNATING §20

segments are overlapped, so as to form a core that provides
a magnetic circuit practically as good as if the disks were
punched in one piece. The use of the dovetail projecting
lugs avoids the use of bolts passing through the disks to hold
the latter in place. Unless bolts are insulated, they are

liable to give rise to eddy cur-
rents by short-circuiting the
disks. Some makers, how-
ever, use disks as shown in
Fig. 18, provided with holes h
for the clamping bolts. The slots used for such stationary
armatures must of course be provided with grooves of some
kind to receive holding-in strips or wedges, as it is not pos-
sible to use band wires in such a case.

45. Revolving armatures are also frequently made of
such large diameter that it is not practicable to punch the
disks in one piece. In such
cases, again, the disks are made
in segments, and are held in
place either by bolts passing
through them or by dovetail
projections fitting into grooves
in an extension of the arma-
ture spider arm. This con-
struction will be understood
by referring to Fig. 19. In F'g- ^»

assembling disks to make up a core, it is usual to place a
heavy sheet of paper about every \ inch or \ inch of core,
in order to make sure that the path for eddy currents will
be effectually broken up.

AKMATURE SPIDERS

46, Disks for revolving armatures are usually supported
on spiders similar to those used for direct-current multipolar
armatures. These spiders are made of cast iron or steel,

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CURRENT APPARATUS

and are necessarily strongly constructed. They should be
so made as to clamp the disks firmly in place, and be amply
strong to bear any unusual twisting action they may have to
withstand due to an accidental short circuit. Fig. 20 shows
two views of a spider and core suitable for disks of moderate
size punched in one piece. The spider proper consists of a

JJ9 Slats

r^}

PtO. 20

hub a provided with four radial arms d that fit the inner
diameter of the disk. The hub is bored out so that it fits
very tightly on the shaft, and a key is provided to avoid
any chance of turning. The core disks d are clamped firmly
in place by two heavy cast-iron end plates c, c that are
pressed up and held by the bolts e. These bolts pass under
the disks, so that there is no danger of their giving rise to
eddy currents. The key / prevents the disks from turning
on the spider and insures the alinement of disks, which is
necessary to make the teeth form smooth slots when the
core is assembled.

Fig. 20 shows the construction used with armatures hav-
ing a small number of heavy armature coils. In such cases

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DESIGN OF ALTERNATING

the coils are stiff and the ends project out past the end of

the core without being supported.
~ In case a distributed winding is
used, the coils are numerous, and
being small, they are frequently
not stiff enough to support them-
selves; hence, the clamping rings
of the spider are in such cases
' provided with flanges, as shown
in Fig. 21. The end connections
of the coils lie on the flat cylin-
drical surfaces a^ a^ and are
tightly bound down in place by
means of band wires. Fig. 22
shows a spider suitable for a
^"°- *^ large armature built up with

segments like those shown in Fig. 19. This style of spider

C— Iru

Fig.

IS common for machines with large diametei of armature

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CURRENT APPARATUS

running at low speeds. The rim r of the spider is made
non-continuous, in order to avoid strains in casting as much
as possible.

47. When the armature is the stationary part of the
machine, a stationary frame of some kind must be used to
support the stampings. This consists usually of a rigid
cast-iron framework provided with end plates, between
which the armature disks are clamped. The construction
will be understood by referring to Fig. 23, which shows a

Pig. 88

stationary armature frame for a machine of large diameter.
The frame casting is usually made in two pieces A and /?,
the lower half being provided with projections a^ a, by which
the spider is bolted to the bed or foundation. The seg-
mental core stampings d, d are held in place by the dovetail
grooves c^ c. These segments are clamped between the end
rings €^ e by means of the bolts /. The end rings e are
shown made up in segments on account of their large
diameter.

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38 DESIGN OF ALTERNATING 8 20

AJEtMATURE CONDUCTORS

48. The style of conductor used on the armature will
depend to a great extent on the current that it is to carry
and the space in which it is to be placed. High-voltage
machines of moderate output are usually wound with double
or triple cotton-covered magnet wire. Frequently two or
more wires are used in multiple in order to secure the requi-
site cross-section. This gives a more flexible conductor than
a single large wire, which would be difficult to wind.

49. It is often advantageous to use bare wire in making
up such conductors and cover the combination of wires with

insulation, as shown in Fig. 24.
A section of a conductor made
up of two bare wires in mul-
tiple is shown at (a), and four
bare wires at (*), the con-
ductors being in each case cov-
ered by the cotton wrapping t. This construction not only
saves space, but the insulation also serves to hold the wires
in place. Conductors of special shape are used on some
machines. For example, square wire and copper ribbon are
often employed. Fig. 24 (c) shows a section of a copper
ribbon conductor with its cotton insulation. Such ribbons
are usually from ^^ inch to ^ inch thick, and should be
made with rounded edges, to prevent danger of cutting
through the insulation.

60. Copper bars are largely used for armatures designed
to deliver large currents. Fig. 24 (d) shows a cross-section
of an armature-winding bar. The dimension // is usually
considerably greater than b, in order to adapt the bar to an
armature slot that is deep and narrow. These bars are
rolled to any required dimensions, the corners being slightly
rounded, as shown, to prevent cutting of the insulation.

Digitized by VjOOQIC

CURRENT APPARATUS

: VI 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 r

^"^""IIIIIIINMIIIIIIII

(a)

illlMIIIIIIIIIIIIJIIlll,lll

IINIIIIIIIIIIIIIIl

(b)

Fig. 25

FORMS OF ARMATURE COIL.S AND BARS

51. The simplest form of coil for alternator armatures
is that used on ordinary single-phase machines with uni-coil
windings. The coils usu-
ally consist of a fairly lar^e
number of turns, and are
wound on forms, so that
the finished coil is of such
shape that it fits snugly
into place in the slots.
Such coils are heavily taped
to insulate them thoroughly
and make them hold their
shape. Coils of this type
are shown in Fig. 25 (a) and (b). The straight portion cc
and dd lies in the slots, the end parts projecting out over the
ends of the armature core. In some cases the ends are curved
as at {a)y while in others the ends shown at (d) are used.

53. In many polyphase windings it is necessary to shape
these heavy coils so that they may
cross each other at the ends of the
armature. This is accomplished by
shaping one of the coils as shown in
Fig. 26. The end of the coil d is
bent down into a different plane
P'^- ^ from that of a, so that the coils

cross each other without touching, and insure good insulation.

53. When coils are used
for a distributed winding like
that shown in Fig. 12, they
are generally shaped like the
coil shown in Fig. 27, which
is the same as those used on
barrel-wound direct-current
armatures. This is a form-
wound taped coil, consisting usually of a comparatively

Digitized by VjOOQIC

DESIGN OF ALTERNATING

FlO. 28

small number of turns. The straight portions a a and bb
lie in the slots, while the end portions project beyond the
core and are usually supported by fianges, especially if the
armature revolves. The side a a lies in a lower plane
than b b^ so that the upper and lower end connections do
not interfere with each other. The terminals /, / of the
coil are usually brought out at the points shown. At the
points r, c the coil is so formed as to bring the end connec-
tions d^d into a plane above ^, a^ and thus bring the side b b
in the top of the slot. Sometimes the terminals are brought
out at the corners a, b, if this brings them in a position
more convenient for connection to the other coils.

54. Bar windings are frequently made in two layers.
Fig. 28 shows a form of bar suitable for a winding such as

that shown in Fig. 13.
The straight part a a
lies in the slot, and the
end portions ^, b form
the connections to the
other bar. Fig. 29 shows one element or turn of such a
winding. The part \ c lies in the top of the slot, and the
two bars making up the
element are soldered
together at the point d.
Fig. 30 shows a similar
element for a wave bar
winding, except that there

is no soldered joint at the "o'^ t>

point ^, the element being ^'o- ^

composed of one continuous copper bar first bent into the
long U form shown in Fig. 31, and then spread out to form
the winding element shown in Fig. 30. Bars of the style
just described are used also for some styles of induction-
motor armatures. The portion of the bar forming the end
connection has to be taped in order to insulate it from its
neighbors. The part in the slot is frequently taped also,
though in some cases the insulation from the core is pro-
vided wholly by the insulating trough.

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j20

CURRENT APPARATUS

Fig. 32 shows a portion of the bar winding on the station-
ary armature of ^^^^^^-^thh^^^

one of the large ifT^^"^ ^^^''^^Ife^^i^^^ _-*-*!&
5,()0()-kilowatt al- JF ^^^^^5i||j^j^(!C>^^

ternators of the ^^^^^^„^

Manhattan Eleva- ^"^^^^^

ted Railway, New ^^^^

York. In this case m

there are three #

bars in each slot, ^

FlO. 80

the bars being first

insulated separately and then bound together. The figure

shows the arrangement of the end connections in two

FIO. 81

different planes, so that they can pass each other with a
good clearance. This armature has a distributed winding

PIO. 88

with 4 slots or 12 conductors per pole per phase. The
armature is Wound for three phases and delivers current
at 11.000 volts.

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42 DESIGN OF ALTERNATING 8 20

ARMATURE INSTXATION (COILS)

55* Alternator armatures are generally called on to
generate much higher voltages than are common with
continuous-current machines. The pressures generated
by ordinary lighting alternators are usually in the neigh-
borhood of 1,000 or 2,000 volts. Power-transmission
alternators with stationary armatures have been built to
generate as high as 10,000 or 12,000 volts. These are the
values of the pressures generated in effective volts, and
when it is remembered that the maximum value of the pres-
sure to which the insulation is subjected is considerably
greater than the effective value, it will be seen that the
insulation of these armatures must be carefully carried out
to insure against breakdowns. The insulation should be
capable of standing a pressure at least three or four times
as great as that at which it is ordinarily worked.

56. For very high-voltage machines it is best to use the
type with stationary armature, as it is easier to insulate a
stationary armature thoroughly. The allowable space for
insulation on a stationary armature is usually greater than
on a revolving one, and, moreover, the insulation is more
likely to remain intact. A revolving armature also necessi-
tates collector rings, brush-holder studs, etc., which have to
be insulated for high pressures; whereas with the station-
ary armature only three terminals are required, which are
comparatively easy to insulate.

57. When the coils each contain a
large number of turns, the voltage gen-
erated per coil will be large; conse-
quently, it is not only necessary to
insulate the outside of the coil thor-
oughly, but each layer must also be
insulated from its neighbor. Fig. 33
shows a section of a coil consisting of
32 turns. Between each layer of wire is a layer of

Digitized by VjOOQIC

§20 CURRENT APPARATUS 43

insulation i turned up at the ends, so as to thoroughly
insulate the individual layers. The whole coil is covered
with a heavy wrapping of insulating tape /, and in Addi-
tion is baked to drive out 'all moisture and treated with
insulating varnish. The thickness of tape will depend on
the voltage of the machine. Linen tape of good quality,
treated with linseed oil, forms about the best material
for this purpose, as it has high insulating properties and
does not deteriorate with a moderate amount of heating
Such tape is usually about .007 to .010 inch (7 to 10 mils)
thick, and is wound on half lapped. Where extra high
insulation is required, the tape may be interleaved with
sheet mica. Coils for distributed windings do not usu-
ally contain a large enough number of turns to require
insulation between the separate layers. They may be
taped and treated with the same materials as the heavier
coils, but the outside taping is usually not so heavy.
With such windings, the material lining the slot is
depended on largely for the requisite insulation.

ARMATURE TNSUL.ATIOX (SLOTS)

68. The taping on the coils is not always depended on
alone for the insulation. The slots are often lined with
insulating material that is not likely to be damaged by
putting the coils in place. Slot insulation is usually made
up in the form of troughs or tubes composed of alternate
layers of pressboard and mica. The mica is depended on
mainly for the insulation, the pressboard being used as a
bonding material to hold the mica in place. These tubes
may be either made up separately or formed in place in the
slots. The mica is usually stuck on the pressboard with
shellac or other insulating varnish, which becomes dry when
hard and makes the trough hold its shape. Fig. 34 shows
the slot insulation for an armature made up of disks similar
to those shown in Fig. 13. The hardwood strip a is first

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DESIGN OF ALTERNATING

§20

laid in the bottom of the slot, and the paper and mica
trough b formed in place before the bonding varnish becomes
dry. The coil r, consisting of several turns of copper wire
or ribbon, is wound in place after the slot insulation has

Pig. 84

become dry, and a wooden wedge d, pushed in from the end
of the armature, holds the winding firmly in place. An
insulating piece e is also placed between the wedge and the
winding.

59. Fig. 35 shows an-
other form of slot insu-
lation; / is the taping on
the coil and i the paper
and mica insulating
trough. The top of the
trough is left projecting
up straight until the coil
is placed in the slot, after
which it is bent over as
shown, protecting the coil
from any injury while the
wedge a is being forced
^^^' * into place. These wedges

should be cut so that the grain of the wood lies across

Digitized by VjOOQIC

§20 CURRENT APPARATUS 45

the slot, otherwise there is danger of their becoming loose
due to shrinkage.

60. Fig. 36 shows the arrangement of slot insulation for
a coil-wound two-layer armature. The in- _
sultating trough i runs around the slot and

laps over the top of the coil as before. In
addition to this, the upper and lower groups
of conductors are separated by the insulating
strip a, which must be sufficiently thick to
stand the total voltage generated. This
arrangement also makes use of the wedge ^^^^
construction for holding the coils in place. fxo. 8«

61. Fig. 37 shows the insulation for a two-layer bar-

■ — ■ wound armature with straight slots. This
style of slot would be suitable for the bar
winding shown in Fig. 13. In such cases the
bars have to be placed in the slots from the
top, the bent ends preventing their being
pushed in from the end. This necessitates
the use of straight slots and band wires for
riG. vi holding the bars in place. A wooden strip is

usually inserted between the band wires and bars in order

to protect the winding.

63. The present practice in armature construction, espe-
cially for high pressures, is to place the itisulation on the
coil rather than in the slot. The coils after being wound
are first thoroughly baked and then placed in hot insulating
compound under pressure, so that the insulating varnish is
forced into the coil. The coil is then taped with several
layers of oiled linen, each layer being treated with varnish
and baked before the next is applied. This gives a dense
hard insulation that offers a high resistance to puncture and
is more homogeneous than the ordinary slot insulation.
The only insulation used in the slot itself is a thin layer of
leatheroid or fiber to prevent abrasion of the coil while it is
being forced into position.

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DESIGN OF ALTERNATING

63.

In using two-layer windings, care should be taken
to have the top and bottom layers very
thoroughly insulated from each other.
The insulating troughs a, Fig. 38, should
project a short distance beyond the
core d, in order to make sure of good

r-^rrr^^ insulation between the coils and core.
'''-' The spider flanges should also be thor-
oughly insulated with paper and mica c

wherever there is any possibility of the

PIO. 88

current jumping from the coils to the spider

MAGNETIC DBNSriTBS

DENSITY IN ARMATURE TEETH

64. Where armatures are wound with a few heavy coils,
the teeth between the coils are large, in some cases nearly
as wide as the pole faces. In such armatures the magnetic
density in the teeth will not be much higher than that in
the air gap. When a distributed winding is used, the sur-
face of the armature is split up more by the slots, and the
area of cross-section of iron in the teeth is reduced. This
gives rise to a higher magnetic density in the teeth than in
the air gap.

65. It was pointed out, in connection with the design of
continuous-current machines, that in such machines it was
desirable to have the magnetic density in the teeth high,
because highly saturated teeth prevent the armature from
reacting strongly on the field and thus aid in suppressing
sparking. In the case of alternators, however, high densi-
ties in the teeth are avoided, because the effects of arma-
ture reaction are not nearly so serious in these machines,
and the high density might prove detrimental by causing
excessive hysteresis and eddy-current losses. In general,
therefore, in alternator design, the magnetic density in the

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§20 CURRENT APPARATUS 47

core teeth is kept as low as possible. The density, however,
cannot be made very low, as this would mean large teeth
and a correspondingly large armature. Where distributed
windings are used, it will generally be found that the width
of the slot and width of tooth are made about equal, thus
reducing the effective iron surface of the armature to about
one-half and making the magnetic density in the teeth about
cwice that in the air gap. It will be remembered that both
the hysteresis loss and eddy-current loss increase very
rapidly with the density ; consequently, it is easily seen that
if the density in the teeth is very high, the amount of loss
in them may be considerable, on account of the high fre-
quency at which alternators usually run. It also follows
that, for the same amount of loss, it would be allowable to
use a higher magnetic density with a low-frequency alter-
nator than with one running at a high frequency.

DENSITY IN ARMATTTIE COBE

66. The density in the armature core proper, that is,
the portion of the core below the armature slots, should
also be low, in order to keep down the core losses. This
density can be made almost as low as we please by decreas-
ing the inside diameter of the core, thus making the depth ^,
Fig. 14, large, and increasing the cross-section of iron
through which the lines have to flow. If, however, the
inside diameter were made very small, the core would be
heavy, and since the hysteresis loss is proportional to the
volume of iron, very little would be gained by decreasing
the density beyond a certain amount. Armature cores for
alternators are usually worked at densities varying from
25,000 to 35,000 lines per square inch, the allowable density
being higher in low-frequency machines than in those run-
ning at high frequencies. Where armatures are run at very
high speeds of rotation, the density may be allowed to run
a little higher than the above values, in order to make the
core as light as possible, provided the frequency is not too
high.

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48 DESIGN OF ALTERNATING

DENSITY IN AIR GAP

67. The allowable density in the air gap will depend, to
a certain extent, on the material of which the pole pieces
are made. If cast-iron pole pieces are used, the density
must be kept fairly low, otherwise there will be danger of
the cast iron becoming saturated. It is best, therefore, to
make the air-gap density in such machines in the neighbor-
hood of 30,000 lines per square inch. If the pole pieces are
made of wrought iron, as they nearly always are in modern
machines, the density may be as high as 40,000 or 60,000 lines
per square inch. The density could be even higher than
this without danger of saturating the wrought iron, but if
the air-gap density is carried too high, a very large mag-
netomotive force must be supplied by the field coils in order
to set up the flux. For these reasons the average air-gap
density should usually be somewhere near the values given
above.

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DESIGN OF ALTERNATING-
CURRENT APPARATUS

(PART 2)

DESIGN OF lOO-KILOWATT SINGLE-
PHASE ALTEBNATOB

!• The general considerations governing the design and
construction of alternator armatures having been given, we
will now apply these to the special case of the design of an
armature for a single-phase alternator, in order to illustrate
the calculation of the different dimensions. As a starting-
point, we will assume that the following quantities are
known, and in this particular case are as given below, the
design being worked out from these quantities. The student
will understand, however, that most of the formulas are per-
fectly general, and that these special values are only taken
to illustrate ,a typical case in order to make the design
clearer. The following quantities are in general known or
assumed: (1) Output at full load ; (2) frequency; (3) speed;
(4) voltage at no load, voltage at full load; (5) allowable
safe rise in temperature; (6) general type of machine.

For the case under consideration we will take the follow-
ing: (1) Output at full load, 100 kilowatts; (2) frequency,
60 cycles per second; (3) speed, 600 revolutions per minute;
(4) voltage at no load = 2,000 = E^ voltage at full load
= 2,200 = E\ (5) allowable rise in temperature, 40° C. ;
(G) general type of machine, belt-driven, revolving arma-
ture, stationary field.

§21

For notice of copyright, see page immediately following the title page.

45—5

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2 DESIGN OF ALTERNATING §21

8. It will be noted that the armature is to deliver
2,000 volts on open circuit and 2,200 volts when the machine
is fully loaded. This is done so that the voltage at the dis-
tant end of the line may remain practically the same from
no load to full load. This increase in voltage is accomplished
by strengthening the field by means of the series-coils, so
that, so far as the voltage generated by the armature is
concerned we design it to generate 2,000 volts, and leave
the increase of 200 volts to be brought about by the action
of the field.

3* Since the speed and frequency are fixed, the number
of poles is also fixed by the relation

where s = revolutions per second;

/ = number of poles;
n = frequency.

We then have

^" 2 ^ 60
/ = 12

and the machine must be provided with twelve poles to give
the required frequency at a speed of 600 {l. P. M. We
might have used a speed of 900 R. P. M. and eight poles,
the frequency being the same in either case. It is better,
however, to use the lower speed (600 R. P. M.) for a machine
of this capacity, so we will adopt the twelve pole 600 R. P. M.
design. The field will be external to the armature, and will
be provided with twelve equally spaced poles projecting
radially inwards. We will also follow the usual practice and
make the distance between the poles equal to the width of
the pole face, or, in other words, make the width of pole face
equal to one-half the pitch. The pole pieces will, therefore,
cover one-half the surface of the armature.

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§21 CURRENT APPARATUS 3

DIMENSIONS OF CONDUCTOR AND COBE

4. The current output at full load will be

, _ watts _ kilowatts X 1,000 .^.

- full-load voltage " T' ^^

100 X 1,000
= 2,200 =^^'^^^P^^^

The machine must therefore be capable of delivering a
current of at least 45.4 amperes continuously without the
temperature rise above the surrounding air exceeding 40° C.

5. The cross-section of the conductor that is used on the
armature is determined by the current that it must carry,
and this in turn depends on the way in which the different
armature coils are connected up. Since the armature under
consideration must generate a high voltage, we will use an
open-circuit winding and connect all the armature coils in
series. The current flowing through the armature con-
ductor at full load will then be the same as the full-load
current output of the machine, that is, 45.4 amperes. The
student should compare this with the calculations determin-
ing the size of wire used on a continuous-current armature.
It will be seen that in this latter case the current in the
armature conductor was less than the total current output
of the machine depending on the number of paths in the
winding. In some of the older types of alternators, the
armature conductors were worked at a high current density,
in some cases less than 300 circular mils per ampere being
allowed. For machines of good design, the number of cir-
cular mils per ampere usually lie between 500 and 700. For
a trial value, take 550 circular mils per ampere in order to
determine the approximate necessary cross-section of the
conductor.

Let

A = area of cross-section of conductor in circular mils;

/ = current in conductor;

m = circular mils per ampere.

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4 DESIGN OF ALTERNATING §21

Then,

A = Im (2)

In this case / = 45.4 and m = 550. Therefore, the cross-
section of the conductor will be

45.4 X 550 = 24,970 circular mils

A No. 6 B. & S. wire would give 26,250 circular mils,
which is quite near to the cross-section required, or two No. 9
wires in parallel would give a cross-section of
26,180 circular mils. Two bare. No. 9 wires
18 covered with a double wrapping of cotton
should be used, because the two wires in mul-
tiple will give a more flexible and easily wound
conductor. The double thickness of this cover-
ing will be about 15 mils. The diameter of No. 9 wire is
.114 inch; hence, the width of the conductor over all will be
.243 inch and the thickness .129 inch. Fig. 1 shows a cross-
section of the conductor, illustrating the arrangement of the
insulation.

DESIGN OF ARMATURE CORE

6. The diameter of the armature is determined by the
speed of rotation and the allowable safe value of the periph-
eral speed. A safe peripheral speed for a belt-driven
machine of this type may be taken at about 5,000 feet per
minute. Hence, the diameter of armature in inches equals

, _ peripheral speed x 12 .^.

"^^ - RTP. M. X ^ ^"^^

5,000 X 12 o, Q . u
= -^r:7T - = 31.8 mches
600 X ^

We will therefore adopt 31 J inches = 31.75 as the outside
diameter of the armature core.

7. The length of the armature core parallel to the shaft,
or the spread of the laminations, must be large enough

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§21 CURRENT APl>ARATUS 6

to enable the armature to present sufficient radiating sur-
face to get rid of the heat generated. In other words, the
armature must be large enough to do the work required of
it without overheating. The core losses and /'-^loss of
the machine under consideration cannot be determined
exactly until the dimensions of the armature have been
determined. The curve shown in Fig. 1, Part 1, gives the
relation between the output and /' R loss for machines of
good design, and it is seen that for a machine of 100-kilowatt
capacity, the P R loss should be about 1.95 per cent, of the
output. The approximate PR loss may then be taken as
100,000 X .0195 = 1,950 watts.

8. This armature is of rather large diameter and runs
at a fairly high peripheral speed. Good ventilation should
easily be obtained by constructing the spider to allow free
access of air and by providing the core with ventilating
ducts. With such an armature there should be no difficulty
in radiating about 2.8 watts for each square inch of core
surface with a rise in temperature of 40° C. The core losses
are apt to be quite large ; hence, to be on the safe side, we
will allow half this radiation capacity for the core losses and
half for the /' R loss. This means that we should have

about — square inch of cylindrical surface for each watt

I* R loss. This would call for a surface of 1,950 X .7
= 1,365.0 square inches.

9. The outside circumference of the armature is 31.75 X^
= 100 inches, nearly ; hence, the approximate length of arma-
ture core parallel to the shaft should be about 13.65 inches.
As a basis for further calculation, we will adopt a trial
length of core of say 14 inches. It may be found necessary
to modify this dimension slightly, as the design is worked
out further, but it should not be made much less than this,
or there will be danger of the armature overheating.

10. We have now determined the approximate dimen-
sions of the armature core, and are in a position to calculate

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6 DESIGN OF ALTERNATING § 21

the magnetic flux 0 after we have decided on the density to
be used in the air gap. This machine will be provided with
wrought-iron pole pieces; hence, we may take 40,000 lines
per square inch as a fair value for the magnetic density in
the air gap. The total magnetic flux ^ from one pole will
be the area covered by the pole multiplied by the mag-
netic density. The poles cover one-half the circumference ;
hence, the length of arc on the armature covered by each
pole will be

number of poles

3.14 X 31.75 X .5
12

= 4.16 inches

The length of the pole face is the same as the length of
the armature core, i. e., 14 inches; hence, the area of the
pole face is 14 X 4.16 = 58.2 square inches.

The total flux from each pole will therefore be 58.2 X 40,000
= 2,328,000 lines.

11. Since the flux^, the frequency », and the E. M. F.
£ generated at no load are now known, the number of
turns T necessary to generate the voltage £ can be calcu-
lated. This armature will be provided with six coils or
twelve slots, that is, one slot for each pole; consequently^
all the conductors may be considered active at once, and we
may use the formula

4.44 * Tn

£ =

10-

^=4.44x<Px;i ^*>

The voltage to be generated at no load is 2,000, the fre-
quency is 60, and the flux 0 is 2,328,000; hence, we have

^ ^ 2,000 X 100,000,000 _
4.44 X 2,328,000 X 60 ""

18. From the above, it is seen that we must place as
nearly 322 turns on the armature as possible. There are

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§21 CURRENT APPARATUS 7

twelve slots, or six coils; hence, there would be ^p

= 53.6 turns per coil and 53.6 conductors in each slot.

This number would not be practicable,

since we should arrange the coils so that

they will wind up into a number of layers

without any fractions of turns. We must

therefore arrange the coils to give the

required number of turns as nearly as

possible, and then modify the length of

the turns, so that the voltage generated

will not be altered. Suppose we arrange

the coil and slot as shown in Fig. 2,

using 8 turns of the twin conductor in

each layer, and having 7 layers per coil. ^^®' '

This will give 56 turns per coil and 56 conductors per slot.

13. The dimensions of the slot may now be determined
from the known number of conductors that are to be placed
in it, and the necessary space that must be allowed for insu-
lation. We will allow .06 inch or 60 mils all around for the
paper and mica tube that composes the slot insulation, and
.04 inch or 40 mils for lapping around the coil. In addition
to this, we will allow for six layers of insulation, 10 mils
thick, between the layers of the coil. This will make the
necessary width of the slot 7 X .129 + 6 X .01 + 2 X .04
+ 2 X .06 = 1.163 inches. The necessary depth of slot will
be 8 X .243 + 2 X .04 + 2 X .06 = 2.144 inches.

In order to be sure that the coil will slip into the slot with-
out having to be forced, and also to compensate for any
slight roughness, we will adopt the dimensions shown in
Fig. 2, namely, 1^ inches wide by 2^ inches deep. We
will make the wooden wedge ^ inch thick, and the opening
at the circumference the same width as the slot, in order to
allow the coil to be slipped easily into place.

14. In order to obtain an even number of turns per coil,
the total number of turns has been increased from 322, as
first calculated, to 336. It follows, therefore, that if the
dimensions of the armature are not altered in any way to

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8 DESIGN OP ALTERNATING §21

compensate for this increase in the number of conductors,
the machine would give more than 2,000 volts when run at
a speed of 600 revolutions per minute. In order, therefore,
to keep the voltage generated the same, each conductor
must be shortened a small amount, so that the poles and
armature core will also be shortened. This will reduce the
flux 0, so that the voltage generated by the 336 conductors
will be 2,000 volts. The final length of armature may be
obtained as follows:

We have * = , 7, ' (6)

4.44 X Tn ^ ^

and in this case

. 2,000 X 100,000,000 _ ^,_. _^^ ,

* = 4.44 X 336 X 60 ^ ^'^^^'^^^' ^^^^^^

That is, in order to keep the voltage the same, the flux is
reduced from 2,328,000 to 2,235,000.
The area per pole will then be

—' A T~ = —^TTT^rTTT- = ^^-^ square inches , (7)

air-gap density 40,000 ^ ^ '

and the length of the pole and armature core parallel to the

shaft will be

area 55.8 ,« 40 • u /q\

— -, = -J-—; = 13.42 mches (8)

polar arc 4.16 ^ '

It will thus be noticed that the armature core is shortened
slightly, thus shortening up each conductor and making the
length of active wire the same with the 336 conductors as it
would have been if 322 had been used. We will therefore
take 13^^ inches as the final value For the length of the core
parallel to the shaft (see /„, Fig. 3).

16. All the essential dimensions of the armature core
have now been determined except the diameter of the
hole in the disks. This inner diameter of the core is
determined by the cross-section of iron that must be pro-
vided to carry the magnetic flux through the armature
core from one pole to the next, and this cross-section in
turn depends on the density at which the core is worked.

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CURRENT APPARATUS

Fig. 3 shows a cross-section of the core, and Fig. 4 shows
a portion of the armature

lying between two pole
pieces. In order to deter-
mine the inside diameter,
we must first obtain the
distance d^, or the depth
of the iron below the bot-
tom of the slots. The
lines of force flow from
the north to the south
pole, as shown in the
figure, and it will be
seen that the number of
lines flowing through
the portion a b under a
slot is one-half the total

j9n%ila/U/ng duet
U 13jj^

K''^'-'--- '"■•■■■'<

PIO. 8

number flowing from the pole

Fig. 4

piece. Hence, the flux through the armature core is \ ^.
The area of cross-section of iron required will then be

_4*

A =

(9)

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10 DESIGN OF ALTERNATING §21

where B^ is the magnetic density at which the core is worked.
We will take the value of B<, as 30,000 lines per square inch.
This will make

^0 = i X gQ J^^ = 37.25 square inches

This is the area of cross-section of iron, and it is equal to

the radial depth of the core under the slots {ab^ Fig. 4)

multiplied by that length of core parallel to the shaft which

is actually occupied by iron. The over-all length of the

core parallel to the shaft is 13^ inches, but part of this

is taken up by the varnish, or other insulation, between the

disks, as well as the portion taken up by the air ducts.

In the present case, we will provide the armature with

three air ducts, each | inch wide, as shown in Fig. 3, the

disks being spaced apart this distance by suitable ribbed

brass castings, or by a special spacing disk. These three

ducts will therefore occupy a linear distance of \\ inches,

leaving 13^V — 1^, or 12^ inches to be occupied by the

iron and insulation on the disks. We will take 11^ inches

as the actual length of iron, the disks being insulated by

having a thin coating of japan placed on every other disk.

37 25
The required radial depth will then be -pp-=- = 3.23 inches.

11. 0

We will therefore make the depth of iron 3^ inches. (See

Figs. 3 and 4.) The total depth of the slot is 2fi^ inches;

hence, the total radial depth of the disk is 2|J + 3^V

= 5| inches, and the inside diameter is 31^ — 2 X 5J

= 20 inches. The dimensions of the disk are, therefore, as

shown in Fig. 4. There are twelve slots of the dimensions

shown in Fig. 2, these slots being spaced equally 30° apart.

CAIiCUIiATION OF ARMATURE L.OSSES

16. The dimensions of the armature having been deter-
mined, it is now necessary to calculate the losses to see if
the armature will deliver the required output without the
losses exceeding the allowable amount. We will first calcu-
late the /• R loss.

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§21

CURRENT APPARATUS

17. The resistance of the armature can be determined
quite closely, since the length of wire on it can be estimated
and the cross-section is already known. The length of wire
can be obtained by laying out one of the coils to scale and
measuring up the mean length of a turn. The coil must
bridge over the distance from the center of a north pole to
that of a south pole, and the ends of the coil must be rounded
out so as to clear the armature core. The coil will be

Fig. 5

shaped as shown in Fig. 5. The straight portion of the coil
will be made 15 inches long, in order to allow the coil to
project about | inch from the slots at each end before it
begins to turn. The mean turn, shown dotted, is the turn
through the center of the coil. Its length is readily deter-
mined from the drawing; in this case it is about 54 inches.
The total length of conductor on the armature will there-
fore be 54 X 336 = 18,144 inches, or 1,512 feet.

18. The hot resistance of any known length of a con-
ductor may be found as follows:

D _ length of wire in inches
'~ area in circular mils

Applying this to the armature just worked out, we find

We will take the resistance as .7 ohm, in order to make
some allowance for the resistance of the connections between
the coils.

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12 DESIGN OF ALTERNATING § ^1

19. The full-load current is 45.4 amperes; hence, the
PR loss at full load will be (45.4)' X .7 = 1,442 watts.
This shows that the PR loss is well under the limit of
1,950 watts and that the armature would be capable of deliver-
ing a little over 45.4 amperes without the PR loss exceed-
ing the allowable amount. The outer cylindrical surface of
the armature as obtained from the final dimensions is tt x 31}
X 13 f'^ = 1,343 square inches, nearly, which allows a little
over .9 square inch per watt PR loss, which should be an
ample allowance for an armature of this type.

20. The hysteresis loss may be calculated when the
volume of iron, magnetic quality of the iron, and fre-
quency are known. The area of the end of the core is
^TT (31.75' — 20') = 477.3 square inches, nearly.

The area of each slot is about 3.4 square inches, and the
total area taken out by the slots 40.8 square inches, leaving
436.5 square inches as the area of the disks. The actual
length of iron parallel to the shaft is 11 J inches; hence, the
volume of iron in the core is 436.5 X 11.5 = 5,020 cubic
inches.

The magnetic density in the core is 30,000 lines per square
inch. Referring to curve B^ Fig. 2, Part 1, we find that for
a density of 30,000 the loss per cubic inch per 100 cycles is
.42 watt. Hence, the hysteresis loss in watts is

21. The eddy-current loss is not easily obtained, but
the combined core losses in this case would likely be fully
as great as, if not greater than, the P R loss of 1,442 watts.
If the combined losses were, say, 3,000 watts, the electrical
efficiency at full load would probably be in the neighborhood
of 94 or 95 per cent., as there would be about 2 per cent,
loss in the field and various connections. The commercial
efficiency would be somewhat less than this on account of
I lie bearing friction, brush friction, etc.

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21 CURRENT APPARATUS 18

ARMATURE WIXDrNG FOR TWO-PHASE
ALTERNATOR

22. The armature just worked out has been designed to
deliver a single current at 2,000 volts pressure. Suppose it
were desired to provide this armature, or rather an arma-
ture of the same general dimensions, with a winding that
would deliver two currents at 2,000 volts pressure, and differ-
ing in phase by 90°. We could use two windings, each con-
sisting of six coils connected in series, the two sets being
displaced GO*' from each other with regard to the poles. The
total output, as before, is to be 100 kilowatts; hence, the
output per phase will be 50 kilowatts, and the current in

t- i_ . r 11 1 J -11 t. 50 X 1,000 ^. „

each phase at full load will be — :ri^ — = 2^- 7 amperes.

The current in the armature conductor is, therefore, one-
half of that in the single-phase machine, and, using the
same current density, we may make the conductor of a
single No. 9 wire instead of two in multiple.

23. The voltage generated in each phase is to be 2,000.
The total magnetic flux is the same, since the size of the
pole pieces and armature is not

altered; hence, the number of con-
ductors in each phase must be 336.
Each coil on the two-phase armature
will therefore consist of 56 turns of
No. 9 B. & S. wire, provided we can
arrange this number satisfactorily in
the slot. If we use 7 layers with
8 turns per layer, we will have a slot
of the same width as before, but only
a little over half as deep. This will
result in a slot that is not very deep fio. 6

compared with its width, whereas it is generally better to
have the slot considerably greater in depth than in width.
It will give a much better proportioned slot if we use only
5 layers, and place 11 turns in each layer, or 55 turns per

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DESIGN OF ALTERNATING

§21

coil instead of 56. This will lower the voltage slightly, but
will leave the dimensions of the core the same, and com-
pensate for this slight decrease by strengthening the field a
small amount. In other words, we will compensate for the
decrease in the number of turns by increasing <P so that E
will remain the same. The slot may then be arranged as
shown in Fig. 6. AJlowing the same amount for insulation
as before, the width of the slot will be equal to 5 X .129 + 4
X .01 + 2 X .04 + 2 X .06 = .885 inch. The depth of the
slot will be 11 X .129 + 2 X .04 + 2 X .06 = 1.619 inches.

We will therefore make the slot ^ inch wide and 1| inches
deep. As this coil is lighter than the one used for the singlp-
phase armature, we will allow only \ inch for the wooden
wedge, and make the upper part of the slot as shown in
Fig. 6. We will leave the inner diameter of the disk the
same, the cross-section of iron being slightly greater than
before, on account of the smaller depth of the slots. The
disk for this two-phase armature will then be of the dimen-
sions shown in Fig. 7. In
this case the disk is provided
with 24 slots of the dimen-
sions shown in Fig. 6, there
being 12 slots for each phase.

24. The PR loss in this
I armature would be practi-
' cally the same as that in the
single-phase . armature pre-
viously calculated. The re-
sistance of each phase will be
about double the resistance
of the single-phase armature,
because in each phase there
is about the same length of wire as before, but this wif-e has
only one-half the cross-section of that used for the single-
phase machine. We may, therefore, take the resistance per
phase as 2 X .7 or 1.4 ohms. The /'A' loss per phase will
be (22.7)' X 1.4 = 721 watts, and the total loss in the two

Fig. 7

Digitized by VjOOQIC

§21 CURRENT APPARATUS 16

phases will be 1,442 watts, as before. The radiating sur-
face has not been altered in any way, so that the two-phase
armature should deliver its output without overheating.
The core losses will also be about the same, because the
volume of the core and the magnetic density have not been
altered materially.

ARMATURE WiNDrNG FOR THREE-PHASE
ALTERNATOR

26« Suppose it were desired to wind the above arma-
ture so that it would deliver 100 kilowatts to a system by
means of three currents differing in phase by 120°. It
would be necessary to supply the armature in this case with
three sets of coils displaced from one another 120° with
regard to the poles. Each set would consist of six coils
connected in series, the three groups being connected
together according to either the Y or A method and the
terminals led to the collector rings. In this case it will be
supposed that the Y method of connection is used, because
the current in each phase is small and the line voltage high.
By adopting the Y method, the voltage to be generated per
phase is reduced, thus calling for a smaller number of turns
per coil than would be required if the armature were A con-
nected. The total output, as before, is to be 100 kilowatts,
and the line pressure at full load, 2,200 volts. We have, for
a three-phase machine,

watts output = ^Z E I

where / is the full-load line current, and E the voltage
between the lines at full load. For the present case, we
have, therefore, 100,000 = 4/3/2,200,

, 100,000

or / = '- — -. = 26.2 amperes

2,200 4/3

86. If the line current at full load is 26.2 amperes, the
full-load current in the armature conductors must also be

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16 DESIGN OF ALTERNATING §21

26.2 amperes, because, in a Y-connected armature, the cur-
rent in each phase is the same as the line current. We will
allow 550 circular mils per ampere, as before, to get an
approximate estimate of the area of cross-section of con-
ductor required. This gives 550 X 26.2 = 14,410 circular
mils.

No. 9 wire has a cross-section of 13,090 circular mils,
while No. 8 has a cross-section of 16,510 circular mils. We
will use the No. 8 wire, since it is on the large side, and will
thus tend to make the /* R loss less. The diameter of this
wire when covered with a double wrapping of cotton will be
about .14 inch.

27. The line voltage at no load is to be 2,000; conse-
quently, the voltage generated in each phase will be '

= 1,154 volts, because the armature is Y connected. We

have

4.44 (PT;.

^ - 10" ^ ^

where E is the voltage at no load generated in each phase.
In this case, the constant k is 1, because we are using a con-
centrated winding, there being only one slot for each pole
and phase. T is the number of turns in each phase. The
magnetic flux ^ will be considered the same as before,
because the dimensions of the pole pieces and armature
have not been altered. We then have

4.44 X ^ X n

^ 1,154X10" ^^. ,

^^ ^ = 4.44x2,235,00-51^50 = ''^ ^^^^^' ^^^^^^

These 194 turns are to be split up into the six coils con-
stituting one phase. We can use 32 turns per coil, and thus
have 192 turns in each phase instead of 194. This slight
decrease in the number of turns could be compensated for
by increasing the field strength slightly. The three-phase

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§21

CURRENT APPARATUS

armature will therefore be provided with 18 coils, each con-
sisting of 32 turns of No. 8 wire. These coils are to be
divided into three sets of six coils,
each of the three sets being con-
nected up Y.

Pig. 8

)^^saattnm

28. The arrangement of the slot

that would probably be best adapted

to this number of turns would be four

layers with eight turns per layer, as

shown in Fig. 8. We will allow the

same thickness of insulation as in

the previous examples, thus making

the width of the slot 4 X .14 + 3 X .01

+ 2 X .04 + 2 X .06 = .79 inch. The depth of the slot will

be 8 X .14 + 2 X .04 + 2 X .06 = 1.32 inches.

We will therefore adopt the dimensions |^ inch by

1| inches as the width and depth, and make the wedge | inch

thick, as in the last case.
Fig. 9 shows the dimensions
of the disk for this machine.
It is provided with 36 slots,
equally spaced and of the
dimensions shown in Fig. 8.
The other dimensions of the
disk remain the same as for
those previously calculated.

29. The /" R loss for this
armature should not differ
greatly from the loss calcu-
lated for the other two. We
can easily make an approxi-
mate estimate of the /' R loss in such a three-phase armature
as follows: The mean length of a turn will be very nearly
the same as that obtained for the single-phase machine,
because the angular distance that the coils span remains the
same and the length of the armature core has not been

45—6

Fig. 9

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18 DESIGN OF ALTERNATING §21

altered. There might possibly be a slight increase in the
length, Qwing to the shape that must be given to the ends
of some of the coils in order to allow them to pass each other
at the ends of the armature, but it will be sufficiently accu-
rate to take the length of a turn the same as before, namely,
54 inches, for the present purpose. The total length of con-
ductor in each phase will be 54 X 192 = 10,368 inches. The
hot resistance of each phase will therefore be

Te^sio^-^^^^^"^

The current in each phase at full load is 26.2 amperes.
Hence the /^ R loss in each phase will be (26.2)' X .628
= 431 watts, approximately. We will take the loss in each
phase at, say, 500 watts, in order to allow for the loss due
to the resistance of the connections. The total loss in the
armature would therefore be 1,500 watts, or about the same
as for the other armatures. The radiating surface is the
same as in the other two cases, so that tiiis armature should
deliver 100 kilowatts within the specified temperature limit.
The core losses, as before, would remain nearly the same,
since the volume of iron has not been changed appreciably.
The coils of the two-phase and three-phase armatures would,
if anything, run cooler than those of the single-phase
machine, because the coils are lighter and the heating effect
is distributed among a larger number of coils.

30. The three-phase armature might have been designed
for a A winding, in which case each phase would be provided
with a sufficient number of turns to generate 2,000 volts.

26 2

The current in the conductor would, however, be only —7—,

or 15.1 amperes; so that, while the number of turns must
be increased, the cross-section of the conductor may be
decreased in the same ratio, and the size of armature slot
will be about the same in either case.

31. The above calculations for single-, two-, and three-
phase armatures have all been made on the supposition that

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§21 CURRENT APPARATUS 19

unicoil, or concentrated, windings were used. The method
of designing the armature when distributed windings are
used is, in general, the same, with the exception that the
formula giving the relation between the E. M. F., flux, and
turns must be modified to suit the style of armature wind-
ing used. The effect of using distributed windings has
already been pointed out, and calculations relating to such
windings will be given in connection with induction-motor
design.

COMPIiETED ARMATURES

32. Fig. 10 shows a finished armature with collector
rings. This armature has a concentrated winding, as indi-
cated by the small number of large slots around its circum-
ference. The wooden wedges for holding the coils in place
are shown at w: c are the ventilating ducts for allowing a
circulation of air through the core. The cast-brass shields J

PlO. 10

are supported from the armature spider, and are used to
protect the projecting ends of the coils. The armature is
shown complete with the collector rings r and the rectifier t.
Fig. 11 shows a large three-phase armature with a distributed
winding. It will be noticed that this armature has a large
number of narrow slots and is similar in appearance to a
continuous-current armature, except for the absence of the
commutator and its connections. The ends of the bars rest

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20 DESIGN OF ALTERNATING - § 21

on the spider flanges and are held down by the bands a.
The disks are carried by the spider b and are clamped up by
the end plates c. The copper bars d^ d are the connections
between the winding and the collector rings. It will be

Fig. n

noticed that this armature is not provided with a rectifier,
because this style of armature is of such low inductance that
the machine can be made to regulate closely enough without
the use of a set of series-coils on the field.

DESIGN OF FIEL.D MAGNETS

33. Stationary field magnets for alternators are gen-
erally constructed in about the same way as those for multi-
polar continuous-current machines, the mam difference
being the large number of poles with which an alternator
field is usually provided. The design almost universally
adopted for stationary fields consists of a circular yoke a^
usually of cast iron (see Fig. 12), provided with a number
of poles d projecting radially inwards toward the armature.
The field is usually made in halves, so that the upper part a
may be removed to give access to the armature. The lower
half b is very often cast with the base of the machine,
especially in machines of moderate size. In larger machines

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§21

CURRENT APPARATUS

the lower half is cast separately and provided with projec-
tions c, c, by means of which it is bolted to the bed. The
halves are held together by means of the bolts e. Some

IRI

PIO. 12

makers build fields of this description, which are divided on
the vertical diameter, allowing the halves to be separated
sidewise in order to get at the armature. In some small
machines the yoke is made in one piece, and the machine is
so arranged that the armature may be
drawn out endwise.

34, The pole pieces used with these
stationary fields are usually straight;
that is, they are not provided with pole
shoes or polar projections of any kind.
Pole shoes are not necessary, because the
length of the polar arc is generally small.
Some of the older types of machines were
provided with cast-iron pole pieces cast
with the yoke, but most modern machines pig. is

have wrought-iron pole pieces built up out of plates and cast
welded into the yoke. Fig. 13 shows a form of cast-iron pole
piece that was used on some of the older machines. This is a
straight pole piece b cast with the yoke a. In order to prevent

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22 DESIGN OF ALTERNATING §21

eddy currents being set up in the pole pieces by the changes
of magnetism in the pole face due to the coarse teeth and
slots of the armature sweeping past it, the surface of the
pole is broken up by a number of thin U-shaped pieces of
sheet iron c cast into the pole. This limits the paths in
which the eddy currents flow, and thus cuts down the heat-
ing of the poles due to them. Cast-iron poles cannot be

^ — -^^^^^ worked at a magnetic density much

U— ?'7\ ^ —^ ^^^^ 30,000 or 35,000 lines per square

•^ inch, and there is always more or less

loss in the polar surface due to eddy
currents. In order, therefore, to do
away with this eddy-current loss and
to permit the use of a higher magnetic
density, laminated wrought-iron pole
pieces have come largely into use, and
are employed on nearly all n»odern
alternators. Fig. 14 shows a common
^'° ^^ form of this type of pole. The pole is

built up of soft iron stampings b, which are clamped together
between the end plates d, d by means of the bolts r, c.
This built-up pole piece is cast into the yoke a. The plates
used for these poles are usually from ^ inch to \ inch in
thickness. If the bolt at the inner end of the pole piece is
very near the end of the pole, it should be lightly insulated
by a paper tube; otherwise it may, by short-circuiting the
plates, allow eddy currents to flow. The length of these pole
pieces parallel to the shaft is made equal to the correspond-
ing length of the armature core. The breadth of the pole w
is determined by the polar arc that the pole must span. It
will be noticed that the cross-section of these pole pieces is,
in general, rectangular, or nearly so, and the field coils are
therefore nearly rectangular. Circular field coils and field
cores, which are so common with direct-current machines,
are seldom met with on alternators, because the width of
the pole lu is generally small compared with the length
of the armature, except perhaps on large slow-speed
machines.

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§21

CURRENT APPARATUS

(a)

35. The yoke a b, Fig. 12, is nearly always made of cast
iron. The magnetic flux through the yoke of an alternator
is usually small, and as
the yoke must have con-
siderable cross.-section to
make it strong enough,
mechanically in any
event, there is no object
in using cast steel to
make the cross-section ^*^- ^5

small, as is frequently done in the case of direct-current
machines. Usually, the yoke is worked at a low density in
order to give sufficient cross-section to make it strong
enough mechanically. The shape of the cross-section is
largely a matter of design, so long as the requisite area of
iron is provided. Fig. 15 {a) shows a plain rectangular
section with rounded corners; {b) shows a section that is
frequently used, the well-rounded corners and the elliptical
back giving the yoke a more graceful appearance than the
plain rectangular section. Fig. 15 {c) shows a section that
is commonly used. In this case the yoke is provided with
flanges that make it stiff and that also give the yoke a
solid appearance, although the cross-section of metal in it
may be quite small (see Fig. 12). Fig. 15 {d) shows a
flanged construction with the flanges moved in from the
edge of the yoke. The breadth of the yoke is usually some-
what greater than the length of the pole pieces parallel to
the shaft, so that the yoke will partially cover the ends of
the field coils.

REVOLVING FIELDS

36. A number of different constructions are used for
revolvliifir fields, depending on the methods adopted for
furnishing the field excitation. A common type is that in
which the radial pole pieces are bolted to a cast-steel rim,
each pole piece being provided with an exciting coil, as
in the case of the stationary field just described. Fig. 16

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DESIGN OF ALTERNATING

§5J1

shows a pole piece and coil for this type of field. The pole a
is built up out of sheet-iron plates and secured by the stud d
to the rim by which is carried on the spokes of the field
spider. Stud d screws into the bar c that passes through
openings in the stampings, and the projections on the pole

PlO. 17

serve to hold the coil in place. In some cases the poles are
made straight and the coil held in place by projecting lugs
on the end clamping plates. Fig. 17 shows a similar pole
piece, the plates in this case being dovetailed into the field
ring and held firmly in place by a key e driven in at one
side.

Fig, 18

37. Revolving fields have been built so as to require
only one exciting coil for all the poles. A field of this type
is shown in Fig. 18. The exciting coil c is circular. The
field casting is in two parts a and /;, held together by boltsy,

Digitized by VjOOQIC

§21

CURRENT APPARATUS

and each casting has a crown of six poles, as shown. When
current is sent through the coil, lines of force thread through
it; all the projections d attached to one side being, say,
north poles, and all those attached to the other side, south
poles. This construction gives rise to large magnetic
leakage, and is now seldom used.

FIELD-MAGNKT COIL.8

38. Field-magnet coils may be wound on spools con-
structed similar to those used for the field coils for continuous-

FlG. 19

current machines. These spools are made so as to slip over
the pole pieces, and are usually held in place by pins pro-
jecting from the pole or by cap bolts screwed through lugs
projecting from the end flanges of the spool. Fig. 19 shows
an end elevation and a cross-sectional view of a spool of the
style commonly used. The shell b is made of heavy sheet
iron, and is flanged up at the ends, so that it may be riveted
or soldered to the brass end flanges ^, a. These flanges are
usually recessed and provided with ribs to make them stiff
and at the same time secure lightness. The ends of the
spool are rounded out as shown, so
as to give clearance for the heads
of the bolts that clamp the pole
pieces together. In designing field
coils and spools, care must be taken
to see that the depth of winding is
not made such that the coils will
interfere with each other when they are placed on the poles,
and sufficient clearance must be j)rovided, as at a. Fig. 20.

Pig. 90

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26 DESIGN OF ALTERNATING §21

39. Field coils are usually wound with double cotton-
covered magnet wire, though in some large machines copper
strip is used. The field spools of most modern revolving-
field alternators are wound with flat copper strip bent on

PlO. 21

edge, as shown in Fig. 21, when (a) represents one of the
laminated pole pieces, with its end insulations. A coil
partly pulled apart is shown at (^). Insulation is placed
between the layers of strip, and the outer edge of the strip

Fig. 22 Pig. 23

is left bare. A coil wound in this way is very solid and
substantial, and the heat is readily radiated because the
exposed strip conducts the heat to the air from the inner
part of the coil. When field coils are provided with two

Digitized by VjOOQIC

§21 CURRENT APPARATUS 27

sets of windings (separately excited and series), the coils
may be arranged on the spool, one on top of the other, as
shown in Fig. 22, or side by side, as in Fig. 23. The con-
struction shown in Fig. 23 is the better, because it admits of
higher insulation and allows one coil to be repaired, in case
of breakdown, without disturbing the other. On many
modern machines the field coils are wound on forms and
held in shape by taping so that it is not necessary to use
spools.

INSULATION OF FIKLX) COILS

40. In many cases the fields are excited by coils that
are provided with only one winding excited from a separate
continuous-current machine. The exciter voltage in such
cases is usually low, and it is unnecessary to take any
unusual precautions in insulating the spools, as the maxi-
mum pressure tending to break down the insulation would
not likely exceed 100 or 200 volts. Such' spools may there-
fore be insulated in the same way as those for ordinary con-
tinuous-current machines.

41. Where the spools are provided with two windings,
the series-winding is, in many cases, in direct connection
with the armature, thus carrying the high potential to the
field coils and subjecting the insulation to a large stress.
Such windings must be thoroughly insulated, not only from
one another, but also from the spools. Figs. 22 and 23
show the methods of insulating these coils. The shell is
covered with several layers a of paper and mica interleaved,
the insulation between the coils in Fig. 22 being also of the
same material. The end insulations b, b and insulation d
between the coils, Fig. 23, are made either of heavy collars
of paper and mica, or of hardwood veneer treated with oil
or other insulating material. Every precaution should be
taken to make the insulation of these spools high, as they
are liable to be subjected to just as high a voltage as the
armature windings.

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28 DESIGN OF ALTERNATING §21

DESIGN OF FIEIiD

42. We will illustrate the method of obtaining the field
dimensions by working out the design of a field suitable for
the single-phase armature previously calculated. This field
will be of the radial pole type shown in Fig. 12, the pole
pieces being of wrought iron, as shown in Fig. 14.

BORE OF POLES AND LENGTH OF AIR GAP

43. Before proceeding with the design of the field, we
must decide on the length of air gap to be used. It was
shown, in connection with continuous-current machines,
that for any given armature it was necessary to have a cer-
tain length of air gap; otherwise, the armature would react
on the field so as to cause sparking when the machine was
loaded. It has also been shown that the general effect of
the armature reaction in an alternator is to weaken the field.
If we wish an alternator to give good regulation, we can cut
down the effect of the armature on the field by using a large
air gap, and on this account it is quite common to find alter-
nators provided with an air gap that is much larger than is
necessary for mechanical clearance. A short gap would
have the advantage of requiring only a small amount of
magnetizing power on the field to set up a given flux; but,
on the other hand, it would allow the armature to react
strongly, the actual length of air gap used not being deter-
mined from considerations of the sparking limit, as it is
in the case of direct-current machines. For belt-driven
machines up to 250 or 300 kilowatts, | inch to | inch may
be taken as fair values for the length of the double air gap.
If the gap is made very large, of course a large amount of
exciting power is required, so that it does not pay to increase
the length of the gap much beyond the values given above.
For large direct-connected machines, the gap necessary for
mechanical clearance will usually be found sufficient to make
the machnie perform well electrically.

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§21 CURRENT APPARATUS 29

44:. For the machine under consideration, we may,
therefore, make the double air gap | inch and the bore of
the pole pieces 31 J + | = 32 J inches. The poles cover
50 per cent, of the armature, and the length of the arc will
be

n X bore of poles X .5

number of poles

(10)

ttX 32.125 X. 5 ,„. ,
or arc = — = 4.2 mches

I/O

The distance between the sides of the pole will be about
4-J inches, as shown in Fig. 24. The length of the pole
piece parallel to the shaft will be the
same as the length of the armature
core, 13^ inches.

45. All dimensions of the pole pieces
are now known except their radial
depth /, Fig. 24. The pole piece must
be made long enough to accommodate
the winding without making it too deep. ^^°- ^

Short pole pieces result in a yoke of small diameter and a
correspondingly light machine. On the other hand, the
spool winding must usually be deep when short spools are
used. The depth of winding may not only be limited by
the space between the poles, but deep windings are objec-
tionable on account of their liability to overheat and the
larger amount of copper required for them. If, however,
the cores are made longer than is necessary, the winding
is made unnecessarily shallow and the yoke of large diam-
eter, thus making the machine heavy and the magnetic
circuit long. In machines of the type under consideration,
the length of the pole piece is usually from 1| to 2^ times
as long as it is wide. For a trial value, we will therefore
take 8 inches as the length /. This can later be increased
or decreased slightly to suit the windings, if found neces-
sary. We will also allow | inch, as shown in Fig. 24, for

Digitized by VjOOQIC

30 DESIGN OF ALTERNATING §21

the thickness of the flat part on the inside of the yoke
against which the coils rest. This will make the inside
diameter of the yoke 32| + 16 + f = 48| inches.

MAGNETIC FLUX THROUGH POUE PIECES AND YOKE

46. The magrnetlc flux that passes through the arma-
ture from one pole piece is ^. A certain number of the
lines leak across from one pole piece to the other without
passing through the armature; hence, in order to get ^ lines
in the armature, we must have ^' lines in the pole piece,
where ^' is equal to ^ multiplied by the coefficient of
leakage. The coefficient of leakage is generally somewhat
greater for alternators than for direct-current machines,
because the poles are usually fairly close together and expose
quite a large surface from which leakage may take place.
The larger the air gap compared with the leakage path
between the poles, the greater will be the amount of leakage,
since the lines always flow by the path offering the least
resistance. The coefficient of leakage also varies with the
size of the machine, being smaller for large machines than
for small ones, and may have values ranging from 2 to 1.3 or
less in very large machines. We will take the coefficient of
leakage for the machine under consideration as 1.4.

47. The useful flux ^ from one pole is in the present
case 2,235,000 lines. The flux through each pole piece will
therefore be <!>' = 2,235,000 X 1.4 = 3,129,000.

The magnetic density in the field cores will be

- flux through core /-i-fx

Of = : (H)

■^ cross-section ^ '

3,120,000 ^^, ^^^ ,. . ^

= Y\ ToT" ~ 56,400 Imes per square mch

It will be noticed that this density is well below that
point at which wrought iron begins to saturate, so that

Digitized by VjOOQIC

§21 CURRENT APPARATUS 31

the sectional area of the pole pieces as determined by
the polar arc is ample for carrying the magnetic flux.

48. The magnetic flux through the yoke is one-half that
through the pole piece, because the lines divide, one half
flowing in one direction and the other half in the other
direction. The number of lines flowing through the cross-
section of the yoke is, therefore,

<P' 3,129,000

Y = -^-i = 1»564,500

and the required cross-section of the yoke will be

. __ flux through yoke __ i ^' /i«>\

"" Allowable density in yoke ~ B^ ^ ^

where B^ is the magnetic density at which the yoke is
worked. The yoke density is usually low, as already
explained, the yoke being made of cast iron. We will take
30,000 lines per square inch a§ the allowable value of Bj,,
thus giving for the required cross-section

. 1,564,500 ^,, . , ,

^ ^ qTwwT" ~ ^^--^ square mches, nearly

We will make the yoke 17 inches wide, so as to allow it to
project over the pole pieces at each end. If we made the
yoke rectangular in sec-
tion, as shown by the
dotted outline. Fig. 25,
the thickness would be
about 3^ inches to give
the requisite cross-sec-
tion. Instead of using
the rectangular shape, we will increase the thickness at the
center to 4 inches and round off the yoke as shown, so as to
keep the area about the same. This will give a heavier-
looking yoke, and one that will present a better appearance
generally than that with a rectangular section.

Fig. 25

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DESIGN OF ALTERNATING

§21

CALCULATION OF FIELD AMPERE-TURXS

49. Since the dimensions of the field frame, armature,
and air gap are now known, and the magnetic densities in
these different parts are also known, the ampere-turns
required to set up the magnetic flux can be calculated. In
order to do this, it is best to consider one of the simple
magnetic circuits shown by the dotted line a-b-^-d-e-f.

Fig. 26

Fig. 26. This path is made up of a portion of the yoke,
two pole pieces, the double air gap, and the portion of the
armature core shown. The dotted line represents the
length of the average path through which the lines flow,
and the ampere-turns supplied by the separately excited

Digitized by VjOOQIC

§21 CURRENT APPARATUS 33

coils on the two poles must be sufficient to set up the mag-
netic flux around this path. We may, for convenience in
making calculations, split up the ampere-turns required for
the whole circuit into the following parts:

1. Ampere-turns required for the double air gap c d-\-ef.

2. Ampere-turns required for the circuit through the
two pole pieces be -\- af.

3. Ampere-turns required for the path through the
yoke a b,

4. Ampere-turns required for the path through the arma-
ture d e,

60. The effective area of cross-section of the air gap
through which the lines ^ flow will be taken as about equal
to the area of the pole face. The lines will fringe to some
extent at the edges of the pole, thus actually increasing the
effective area slightly. The area is, however, cut down
somewhat by the air ducts in the core, so that this will tend
to counterbalance any increase in area due to fringing. We
will therefore assume that the density is as taken at the out-
set, namely, 40,000 lines per square inch. The permeability
of air is 1, and the total length of air gap is | inch; hence,
ampere-turns required for double air gap = H X /x .313
= 40,000 X .375 X .313 = 4,700, nearly.

51. The magnetic density in the pole pieces has already
been determined and found to be 56,400 lines per square
inch. The length of path through the two pole pieces
is 2 X 8 = 16 inches. By referring to the magnetization
curves. Dynamos and Dynamo Design^ Part 2, we find that
it requires about 11 ampere-turns per inch of length to
set up a density of 56,400 lines per square inch through
wrought iron. Hence, ampere-turns required for field
cores = 11 X 16 = 176.

53. The yoke has been made of such cross-section that
the density in it is 30,000 lines per square inch. The length
of the path ab through the yoke can be scaled from the
45—7

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34 DESIGN OF ALTERNATING §21

drawing, and in this case is about 14| inches. For a den-
sity of 30,000 lines per square inch, the ampere-turns
required per inch of length for cast iron are about 50.
Hence, ampere-turns required for yoke = 50 X 14^ = 725.

53. The armature has been made of such cross-section
that the density in the core is about 30,000 lines per square
inch. The length of the path through the core can be
obtained from the drawing; in this case it is about 12 inches.
The ampere-turns required per inch of length for wrought
iron at this density will be about 8. Hence, ampere-turns
required for armature core = 8 X 12 = 96.

54. The total ampere-turns that must be supplied by
one pair of the separately excited field coils will be the sum
of the ampere-turns required for the different parts of the
magnetic circuit; hence, total ampere-turns = 4,700+176
-h 725 + 96 = 5,697, say 5,700.

The student will note that because the magnetic densities
in the iron parts of the circuit are low, and also because the
lengths of the different paths are short, the ampere-turns
required for the iron part of the circuit are small compared
with those required for the air gap, which has a high mag-
netic reluctance. The ampere-turns required for the arma-
ture core might in many cases be neglected without serious
error. It follows from this that if it is found necessary
later to lengthen or shorten the pole pieces slightly, in order
to accommodate the winding, the corresponding resulting
change in the ampere-turns will not be appreciable.

CALCULATJON OF SEPARATELY EXCITED WINDING

65. Having determined the ampere-turns to be supplied
by each pair of separately excited coils, the next step is
to design a winding for these coils that will supply the
required number of ampere-turns. The size of wire can
readily be determined when the mean length of a turn and

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§21 CURRENT APPARATUS 35

the voltage across the coils are known. In order to get
at a value for the mean length of a turn, we must adopt a
trial value for the depth of the winding. Suppose we make
the spool flanges 1^ inches deep, as this will give a spool of
dimensions well suited to the field shown in Fig. 26, allow-
ing plenty of clearance space between the coils when they
are slipped over the poles. The clearance between the shell
and field core will be, say, ^ inch all around, and we will
allow ^ inch on each side for the thickness of the shell and
insulation. The series and separately excited coils will be
arranged side by side, as shown in Fig. 23. We will have
a clear depth of winding of 1 inch, allowing for clearance
and insulation as above. The shape of the spool will be as
shown in Fig. 19, and the mean length of a turn can readily
be measured off the drawing. In this case the mean length
of a turn will be about 41 inches, or 3j^ feet.

56. The separately excited coils are connected in series,
so that the voltage across any pair of coils will be the volt-
age across all the coils divided by the number of pairs of
poles on the machine. The voltage applied to the separately
excited field is equal to the voltage generated by the exciter
less whatever drop there may be in the regulating rheostat.
Let E represent the E. M. F. generated by the e/.citer, and
e the drop in the rheostat. The pressure applied to one pair
of coils will then be

I
2
where/ = number of poles;

The current in the field will be
where R is the resistance of a pair of spools.

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36 DESIGN OF ALTERNATING §21

But the hot resistance R of a, pair of spools may be
expressed as follows :

R = :^??L>^ (14)

where /« = mean length of a turn in inches;

T = number of turns on a pair of spools;
m = circular mils cross-section of field wire.

Substituting in formula 13 the value of R as given by
formula 14, we get

. ^ li^JZfl^ (15)

(16)

The values of the quantities T and / are not known sepa-
rately, but their product is known, since it is the ampere-
turns supplied by one pair of spools. Hence, we may write

circular mils cross-section of separately excited field wire

_ number of poles X mean length of a turn in inches X ampere-turns
~ 2 (voltage of exciter — drop in field rheostat)

Or, the cross-section in circular mils of the wire necessary
for the separately excited winding of an alternator is found
by taking the product of the number of poles, the mean
length of a turn in inches, and the ampere-turns supplied
by one pair of spools, and dividing by twice the voltage of
the exciter less the drop through the field rheostat.

The size of wire could be worked out equally well by con-
sidering the ampere-turns supplied by all the coils instead
of a single pair, and taking the total voltage instead of the
voltage across a pair of spools. It is best, however, to make
the calculations with reference to a pair of spools in order
to avoid confusion, because the ampere-turns were calcu-
lated for a pair of spools.

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§21 CURRENT APPARATUS 37

67. The exciter voltage E is commonly 110 volts, though
other voltages are sometimes used with large machines.
The use of 110 volts is common, because it permits the use
of an ordinary 110-volt incandescent dynamo as an exciter.
We will assume that the field for which we are making cal-
culations is supplied from a 110-volt exciter, and that the
normal drop in the rheostat is 10 volts. This will make the
pressure across the twelve field coils 100 volts total. We
then have

. , .. 12X41X5,700 ,^^^^

circular mils = — — — '- = 14,022

/vOO

The nearest size to this is No. 9 B. & S. having a cross-
section of 13,090 circular mils. \Ve will therefore adopt this
size of wire for the separately excited field, the slight differ-
ence in cross-section being compensated for by cutting out
a little of the rheostat resistance.

58. The current density in the field should be consider-
ably lower than ir the armature, because the field windings
are deeper and the heat is not so easily dissipated. The
current in the separately excited winding is about the same,
no matter what load the alternator is carrying, and in this
respect is not like the current in the series-coils, which varies
with the load. For these reasons, it is not safe to allow
much less than 1,000 or 1,200 circular mils per ampere in
the separately excited winding, and in cases where the wind-
ing is very deep a larger allowance than this may be required.
In the present case we will take 1,100 circular mils per
ampere as a fair value, thus limiting the current to VtVo^
= 11.9 amperes.

59. With a field current of 11.9 amperes, the number of

turns required per pair of spools will be ' = 478 turns,

nearly. Each coil should then have 239 turns of No. 9
B. & S. double cotton-covered wire. The diameter of this
wire over the insulation will be about 120 mils, and if the
coil is wound in eight layers, the depth of winding will be

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38 DESIGN OF ALTERNATING §21

1.008 inches, so that an eight-layer winding will fit the
1-inch winding space on the spool. If we use thirty turns

to a layer, we will have
240 turns per spool. This
is an increase of one turn
over the number actually
required, but it will be
better to use this winding
than to have an uncom-
pleted layer, since the
difference is so small.
The length of -winding
space occupied by the
coil will be 30 X .126
= 3.78 inches, or, say,
3| inches, so as to be
^^®- ^ sure of enough room.

The separately excited coil will therefore be wound with
eight layers of No. 9 wire with thirty turns per layer, the
winding space occupied being 3| inches long and 1 inch
deep. The use of 240 turns per spool, instead of 239 turns,
will not affect the current appreciably. The upper coil 5,
Fig. 27, shows the arrangement of this coil on the spool.

COMPOUND, OR SERIES-FIELiD, WINDING

60. The compound winding must provide a sufficient
number of ampere-turns to compensate for the falling off in
voltage at the terminals due to the resistance of the arma-
ture and the combined effects of armature inductance and
armature reaction. The compound winding must also pro-
vide the ampere-turns necessary for any increase in terminal
voltage in cases where the machine is to be overcompounded.
The calculation of the compound winding depends to a large
extent on data obtained from machines of a similar type.
Its determination for a machine of new type is always -more
or less experimental.

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§21 CURRENT APPARATUS 39

61. The current that is led through the series-winding
is first rectified, as explained in former articles, and as the
current increases in proportion to the load, the field is
strengthened proportionally, provided the. magnetic circuit
is not saturated. This is usually the case with alternators,
so that we may assume that any change in the field current
is accompanied by a corresponding change in the field
strength. It is not usual to send the whole of the current
around the series-fields; part of it is shunted through a
German-silver resistance', by varying which the amount of
compounding can be varied. This allows a considerable
adjustment of the series-coils, so that their effect on the
performance of the machine can be varied through a wide
range^ without changing the series-winding in any way.
Sometimes the whole current is not rectified, a portion of it
being shunted around by means of a resistance connected
to the two sides of the rectifier. In this case the shunt
must revolve with the armature, and is usually mounted on
the armature spider. Revolving shunts are generally used
on machines of any considerable size, as they avoid the
difficulty of commutating a large current. Compound coils
are only necessary on the fields of machines that have high
armature inductance or resistance, or on machines that
must give a considerable rise in voltage from no load to full
load. Other types of machines can be made to give suffi-
ciently good regulation by the use of separately excited coils
only. Most of the alternators of large output installed in
modern power plants are plain separately excited machines.

• S2. The drop due to the resistance of the armature is
easily calculated when the armature resistance is known, as
it is equal to the product of the armature resistance and
the full-load current. In this case, therefore, the armature
drop will be 45.4 X .7 = 31.78 volts.

63. The machine is to supply 2,000 volts at no load and
2,200 volts at full load; the compound winding must there-
fore strengthen up the field sufficiently to generate this

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40 DESIGN OF ALTERNATING §21

200 additional volts, as well as the 31.78 volts required to
overcome the resistance of the armature. If there were no
armature inductance or armature reaction, the total volts
that would be generated at full load would be about 2,232.
The ampere-turns supplied by two separately excited coils
(i. e., 5,700) are sufficient to generate 2,000 volts; hence, if
the above conditions were attained, the ampere-turns on
the field at full load would have to be ||ff X 5,700 = 6,361,
and the ampere-turns that would be supplied by the series-
coils would be 6,361 — 5,700 = 661, or about 331 on each
spool. For a machine, of this kind, however, this would
represent only a very small part of the series ampere-turns
that would actually be required, because, in the first place, •
the field is weakened by the reaction of the armature,
and, secondly, a large E. M. F. has to be generated to
force the current through the armature against its induct-
ance. In machines of this type the compound ampere-turns
may be as much as two-thirds or more of the ampere-turns
supplied by the separately excited coils. In the present
case, therefore, we will design each spool so that it will
be capable of supplying about 2,600 ampere-turns. If this
should prove to be somewhat more than is actually required,
it can easily be cut down by allowing more current to flow
through the shunt.

64. We will assume that 70 per cent, of the current at
full load flows through the series-coils, the remaining 30 per
cent, flowing through either the revolving or stationary
shunts. This will make the current in the series-coils
45.4 X .70 = 31.78, say 32 amperes, nearly. The number
of turns required for each series-coil will then be ^jp
= 78.4 turns.

65. The current density in the series-coils should be
about the same as that in the separately excited windings.
If we allow 1,100 circular mils per ampere, as before, we
get a cross-section of 32 X 1,100 = 35,200 circular mils.
Two No. 8 wires in parallel give 33,020, while two No. 7

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§21 CURRENT APPARATUS 41

wires give 41,640. We will adopt the conductor made up of
two No. 8 wires, because the current in the series-coils is
not apt to be continuously at 32 amperes, and we can there-
fore afford to use a cross-section that is a little on the small
side. The outside diameter of No. 8 wire with cotton insu,
lation is about .140 inch; hence, in a winding space 1 inch
deep we can place seven layers. If we use 11 turns per
layer, we will have 77 turns per coil, and can compensate for
the slight decrease in the calculated number of turns (78.4)
by changing the shunt a little, so as to cause a correspond-
ingly larger amount of current to flow through the coils.
Each turn consisting of two wires in parallel will occupy a
length along the winding space of .280 inch, and 11 turns
will take up a space of .280 x 11 = 3.080 inches, say
3^ inches. We will allow ^ inch at each end and between
the coils for the hard-wood insulating collars, thus making
the total axial length taken up by the windings and
insulation 3| + 3| -|- ^ = 7iV inches. The brass flanges
on the spools will be about { inch thick, so that the
total space taken up on the pole piece will be 7^^ + i
= 8^ inches. The radial length of the pole piece as
originally assumed was 8 inches; it will therefore be
necessary to lengthen out the poles a little, in order to
accommodate the spool, and * increase the diameter of
the yoke correspondingly. It is best to have the pole
project beyond the spool flange a little, as it keeps the
flanges away from the armature and makes it easier to
fasten the spools in place. We will therefore make each
pole piece 8J inches long instead of 8 inches. Fig. 27 shows
a section of the spool with both windings in place. The
pole piece is indicated by the dotted outline. This change
in the length of the pole piece will make the inside diameter
of the yoke 49| inches, and the outside diameter 57| inches,
as shown in Fig. 26, where the final dimensions are encircled
by rings. The spools are held in place on the poles by
pins (not shown in the figure), which are fixed in the
pole pieces so as to prevent the coils slipping down on
to the armature.

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42 DESIGN OF ALTERNATING §21

liOSS IN FIEIiD COLLS

66. The loss In the field coils should be determined,
in order to see if sufficient radiating surface is provided to
dissipate the heat. The resistance of the twelve separately
excited coils will be

^ _ 12 X 240 X 41 ^ , . ^ ,

• 13 09Q ~ ^ ohms, approximately

since there are 240 turns on each spool.

The i^ R loss in the separately excited coils will therefore
be (11.9)'' X 9 = 1,274 watts.

67. The resistance of the twelve series-coils is

^ 12 X 77 X 41 , _ ,
^^=— 33:020— =^-^^^^"^'

The I* R loss in the series-coils will therefore be (32)*
X 1.15 = 1,178 watts, nearly.

68. The total loss in the field will be 2,452 watts, or
about 2.4 per cent, of the output. This is the maximum loss
when the machine is working at its full output. The
average field loss would probably not be over 2 per cent, of
the output, as the loss in the series-coils would not be as high
as -1,178 watts all the time. The loss per coil will be ^f|^
= 204 watts. The surface of each coil (not counting the
ends) is about 350 square inches. This area is obtained by
multiplying the perimeter of the coil as obtained from the
drawing by the length of the coil along the pole piece.
This area gives an allowance of 1.7 square inches of surface
per watt, which is sufficient to insure a rise in temperature
not exceeding 40° C. As far as heating goes, the design of
the winding is therefore satisfactory.

69. The curve shown in Fig. 28 gives the relation
between the average field PR loss and the output for

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§21 CURRENT APPARATUS 43

alternators of good design. For a 100-kilowatt machine the

S€laiion between field I*B io§§ and output of tUiematwr^

Fig. 28

average loss is about 1.7 per cent., which is slightly lower
than that for the machine just calculated.

MECHAiaCAX. CONSTRUCTION

FIELD FRAME ANO BED

70. Fig. 29 shows the field frame, with bed and bear-
ings, for the machine designed, and will serve to illustrate
the general method of construction used for machines of
this type. In this case, the field is shown as a separate
casting bolted to the base, but, as mentioned before, many
machines are constructed wMth the lower half of the field
cast with the base. Where the machine is of large size, it
becomes difficult to cast the field and bed together, and
the construction shown is usually adopted in such cases.
The field is usually set down into the bed, as this
lowers the center of gravity and tends to make the

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§ 21 CURRENT APPARATUS 45

machine run steadier. The distance between the centers
of bearings is determined by the over-all length of the
armature and the space taken up by the collector rings.
The bed itself is almost exactly similar to the beds used for
multipolar continuous-current machines; it is made hollow
and provided with ribs to insure stiffness. The thickness of
metal in the bed will vary from about J inch or | inch up
to IJ inches or 1^ inches for machines varying in size
from about 50 to 500 kilowatts. Self-oiling bearings of
the ring type are used almost exclusively. The bearing
pedestals, as shown in Fig. 29, are cast with the base, though
in many large machines it is common practice to cast them
separately and bolt them to the bed. The bearing cap and
pedestal is grooved at a a to receive the rocker-arm, which
carries the rectifier brushes. Some makers place the recti-
fier and collector rings outside the bearing and bring the
connecting wires through the shaft ; in such cases the out-
side end of the bearing cap and pedestal must be grooved to
receive the rocker-arm. Machines of the type shown are
usually arranged so that they can be mounted on rails in the
same manner as continuous-current machines.

COLLECrOU RINGS AND RECTIFIER

71. One of the distinguishing features of an alternator is
the arrangement by which the current is collected. The
commutator of the continuous-current machine, which is
usually made up of a large number of parts, is replaced, in a
simple alternator, by two or more plain collector rings. In
case the alternator is compound-wound, the commutator is
replaced by two or more collector rinp^s in combination
with a rectifier. Although there are, in general, a small
number of parts connected with a collector as compared
with a commutator, the mechanical construction of the col-
lector must be carefully carried out, because it is often
necessary, where revolving armatures are used, to secure
high insulation. Fig. 30 shows a Construction that may be

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DESIGN OF ALTERNATING

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§21 CURRENT APPARATUS 47

used for simple collector rings. Such a pair of rings would
be suitable for a single-phase alternator with a separately
excited field winding only. The same construction could
be used for separately excited two-phase or three-phase
machines, the only difference being in the number of rings
employed. The rings r, r are made of cast copper, which
must be free from blowholes or imperfections tending to
cause uneven wear. . These rings are usually made heavier
than is necessary for collecting and carrying the current,
in order to make them strong mechanically and to allow
for wear. Fig. 30 shows the construction used for rings
that are subjected to a pressure of about 2,000 volts.
The rings are cast with a hub ^, which supports the rings
by means of the spokes c. The insulation d between the
disks is usually made of either red fiber or hard rubber,
the latter being preferable, especially for high potentials.
These insulating disks should be at least \ inch thick, in
order to keep them from breaking easily, and they should
also project some distance above the surface of the rings, in
order to avoid any danger of the current arcing over from
one ring to the other. The insulating washers and collector
rings are assembled on a shell e, made either of cast iron or
brass, the latter being preferable for collectors of small
size. This shell is thoroughly insulated with several layers
of mica, and the assembled collector is clamped firmly in
place by means of the nut /and washer^. When the col-
lector is of large diameter, it is usually clamped up by
means of bolts instead of the nut/" The connections to
the rings are made by two copper studs //, which pass
through the back of the shell and connect to each of the
rings by being screwed into one of the spokes, as shown.
These studs are heavily insulated throughout their length
by tubes made of mica or hard rubber. After the ter-
minals of the armature winding have been attached to the
studs, all exposed parts should be heavily taped to avoid
any danger of arcing from one terminal to the other.
Where the studs pass through the back of the shell, they
are insulated by thick hard-rubber bushings k.

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48 DESIGN OF ALTERNATING §21

73, The dimensions of the rings are determined quite
as much by mechanical considerations as by the current
that they are to collect. The surface of the rings should be
wide enough to present sufficient collecting surface, and
they should be thick enough to allow for a reasonable
amount of wear. Such rings should collect at least
200 amperes per square inch of brush contact surface.
This assumes that copper brushes are used, which is often
the case with alternators. The freedom of carbon brushes
from cutting and their better performance generally have
resulted in their being used largely on alternators, though,
of course, their advantages as regards the suppression of
sparking do not have the force here that they do with
direct-current machines. Carbon brushes require about
three times as much contact surface, for a given current,
as copper brushes, and this large collecting area is usually
obtained by using a number of brushes distributed around
the circumference of each ring, instead of increasing the
width of the ring itself. The rings should not be made of
too large diameter, or the rubbing velocity between the
brush and ring will be high, thus tending to cause uneven
wear and cutting. On the other hand, if the rings are
made of very small diameter, they must be made wide to
present sufficient collecting surface, thus necessitating the
use of wide brushes. If a large collecting surface is
required, it is best to use a ring of moderately large diam-
eter, and use several brushes on each ring. From 1,500
to 2,500 feet per minute are fair values for the peripheral
speed of collector rings for belt-driven machines. The
rings shown in Figs. 30 and 31 are 10 inches in diameter.

On large revolving-field alternators, the collector rings
are usually made of cast iron instead of copper. This is
much cheaper, and it is found that carbon brushes bearing
on cast-iron rings give excellent results, the iron ring taking
on a good polish. On these large machines, the collector
rings are usually made in halves, suitably fastened together,
so that the rings may be put in place or removed without
disturbing any of the heavy parts of the alternator.

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§21

CURRENT APPARATUS

46—8

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DESIGN OF ALTERNATING

§21

73, For compound-wound machines, it is necessary to
have a rectifier in addition to the collector rings. The
rings and rectifier are usually built up together, though
some makers mount them on the shaft separately. Fig. 31
shows a combined pair of collector rings and rectifier suit-
able for the single-phase machine designed. The rings are
made 10 inches in diameter and 1^ inches wide, the con-
struction used being the same as that already described.
The rectifier is made up of two castings, each having
six sections, those belonging to one casting being marked a,
and those belonging to the other, b. These two castings
are separated by the mica collar r, while mica insulation is
provided between the segments a and /;, as in a regular con-
tinuous-current commutator. One set of segments connects
to one of the collector rings through the hubs, as shown
at d. The other rectifier casting is connected to the stud e^
which is, in turn, connected to one terminal of the armature
winding. The other stud is connected to the remaining
collector ring. The details of construction will be under-
stood by referring to the drawing, as they are almost
identical with those described in connection with Fig. 30.

BRUSHES AND BRUSH HOLDERS

74, Copper brushes are generally used on the smaller
sizes of alternators, and copper leaf or wire brushes similar

to those used for di-
rect-current machines
are employed on many
machines, though
carbon brushes are
now largely used on
account of their
superior wearing
qualities. It is best
to have at least two
brushes for each col-
essential as with

Fig. 82

lector ring, though this is hardly as

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§21 , CURRENT APPARATUS 51

direct-current machines, because collector-ring brushes do
not need as much attention while the machine is running as
those used with commutators; for this reason, a large num-
ber of machines are built with only one brush for each
collector ring. Two or more brushes should, however, be
used for each terminal of the rectifier, because these
brushes are liable to need more or less adjustment while
the machine is running. The holders used should be so
designed that the copper brush will press on the rings at
an angle of about 45°. Any good form of copper brush
holder used on continuous-current* machines will answer
equally well for an alternator. Such a holder should be
arranged so that the brushes may be lifted from the com-
mutator and held off, and the pressure of the brush on the
ring should be* easily varied. The pressure of the brush on
the ring may be provided by making the brush itself act as
a spring, or the holder may be provided with a spring, the
tension of which is adjustable. Fig. 32 shows a simple type
of holder that has been used considerably on alternators.
The brush is made long enough between the holder h and
the ring r to render it flexible and allow it to follow any
unevenness of the surface. The pressure on the ring can be
varied by changing the position of the holder on the stud
by means of the clamp 5. One advantage of this style of
holder is that the current has no loose contact surfaces to
pass through between the brush to the brush-holder stud.
The carbon brush holders used on alternators are similar to
those used on direct-current machines and require no special
description.

BRTTSH-HOLBER STUDS

76, Brush-holder studs follow the same general design
as those used for ccmtinuous-curi ent machines, special care
being taken to have them very well insulated. Fig. 33
shows a common type of stud and the method used for
insulating it. The brass stud a is circular in cross-section
and is provided with a shoulder g that clamps against a

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DESIGN OF ALTERNATING

§21

washer //. The stud is- insulated from the rocker-arm
by a heavy hard-rubber bushing / and washers b. The
bushing / is let into the washers ^, as shown, in order
to break up the path by which the current tends to

Fig. 38

jump from the stud to the supporting casting. The sharp
corners of the casting should also be removed, as shown
at m. The cable terminal d is clamped between the
washer c and the nut e. Fig. 34 shows another method
that is sometimes used for mounting and insulating brush-
holder studs. A hard-rubber tube a fits tightly over the

FlO. 34

stud b and completely covers it except at the points where
the brush holders and cable connections are placed. The
brush-holder stud is clamped to the rocker-arm, as shown,
by means of the cap c and the cap bolts d. Connection is

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§21 CURRENT APPARATUS 53

made to the cable at the end of the stud. This construc-
tion gives very good insulation between the stud and the
rocker, because the insulation is unbroken and no path is
open for the current to jump across unless it punctures the
tube itself.

76. The studs that carry the rectifier brush holders
should be mounted on a rocker-arm, so that they may be
adjusted, with reference to the
field, in the same manner as the
brushes of a direct-current ma-
chine. The studs for the collector-
ring brushes may be carried on
the same rocker-arm, or may be
mounted on a stationary stand
bolted to the bed of the machine.
The collector-ring brushes do not need to occupy any
definite position relative to the field; hence, it is not
necessary that they should be mounted on the rocker-arm,
though this is very often done for the sake of convenience
and cheapness of construction. The angular distance
between the arms of the rocker carrying the rectifie.r studs
will depend on the number of poles on the machine. Sup-
pose Fig. 35 represents the rectifier for the twelve-pole
machine worked out. All the light sections belong to one
casting and the dark ones to the other. The angular dis-
tance from center to center of segments is 30°. When one
set of brushes is on a light segment, the other set must be
on a dark segment; hence, the brushes might occupy the
position cd'. This, however, would bring the brushes too
close together, and we will place the rocker-arms so as to
make them as far apart as possible, and still have them
conveniently located. We will therefore place the rocker-
arms carrying these brush-holder studs 150° apart, thus
bringing the brushes into the position c d,

77, Fig. 36 shows a rocker-arm suitable for the single-
phase machine designed. The arms a^ b are 150° apart, and

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DESIGN OF ALTERNATING

§21

carry the rectifier studs, the arms c, 3 for the collector-ring
studs being carried on the same rocker. The hub^ is bored
to fit the groove in the bearing cap, and the rocker is made
in halves, as shown, so as to be easily removable, and held

Pig. 80

kFJ

together by bolts ;f,^. The lug /is tapped out to receive a
handle, which serves both to shift the rocker and clamp it in
any desired position by screwing it down against the seat on
which the rocker moves.

SHAFTS

78. Shafts for alternators are designed a<:cording to the
same rules as those for direct-current machines. These
shafts are usually made larger than the size called for by
the power to be transmitted. Stiffness is an essential fea-
ture of all armature shafts, and in order to secure this, they
are made quite large, considering the actual amount of
power that they must transmit. This is necessary, because
the shaft must not only support the weight of the armature,
but it may also be called on to stand heavy magnetic pulls
if the field is not evenly balanced. A shaft suitable for the

Digitized by VjOOQIC

§21

CURRENT APPARATUS

100-kilowatt machine is shown in Fig. 37.
for a pulley journal, 13 in. X 4 in., and
journal, 10 in. x S^ in. The keyway a is
spider key. The central portion of the
spider fits on is usually made a little large,
may be forced into place. The keyway
shown at d. All internal corners of the

This is designed

a collector end
for the armature

shaft where the
so that the spider
for the pulley is

shaft should be

Fig. 87

rounded, as shown at c^ c, and oil grooves d, d should be
provided to prevent the oil from working its way out of the
boxes by creeping along the shaft. In many cases the
exciter is driven from a pulley mounted on an extension
of the armature shaft. The shaft must then be furnished
with a keyway on the extension for the exciter pulley, as
shown by the dotted lines.

PULI.EY8

79, Ordinary cast-iron pulleys are usually employed.
Broad-faced pulleys are usually provided with two sets of
arms, and the pulleys, on the whole, are constructed some-
what heavier than those used for general transmission work.
Large pulleys should be made in halves, and strongly bolted
together both at the hub and rim. The diameter of the
pulley is determined by the linear speed at which it is
allowable to run the belt. . A fair average value for this
belt speed may be taken from 4,000 to 5,000 feet per
minute for machines varying in size from 50 to 500 kilo-
watts. It is not advisable to run the belt at a speed much
higher than 5,500 feet per minute, as the grip between the
belt and pulley becomes less with higher speeds. The diam-
eter of the pulley in inches is then given by the expression

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DESIGN OF ALTERNATING

§21

12 5'

diameter of pulley = ^^ ^^ ,, (17)

^ ^ TT X R. P. M. ^ '

where 5' = belt speed in feet per minute.

Applying this to the 100-kilowatt machine, and taking
4,500 feet per minute as a fair value for the belt speed, we
get

diameter of pulley = -— - — ^.-- = 28.6 inches
^ ^ 3.14 X GOO

We will make the diameter of the pulley 28J inches, as
shown in Fig. 38. The face of the pulley must be slightly
wider than the belt necessary to transmit the given amount
of power at the required belt speed. The belt must be of

PlO. 88

such width that the strain on it per unit width will not be
more than the belt can safely carry. The amount of power
that can be transmitted per unit width of belt depends on
the quality and thickness of the belt as well as on the belt
speed. Assuming that a double thick belt is used, we may
determine the width of belt necessary by means of the fol-
lowing formula;

width of belt = .7 X

(18)

where W = output of generator in watts.

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§21 CURRENT APPARATUS 57

Applying this to the 100-kilowatt machine, we get

'ji.u r u 1. w 100,000 , ^ ^ . ,

width of belt = .7 X ^ * ^ = 15.5 inches
4,500

We will allow | inch on each side of the belt, thus making
the face of the pulley 17 inches wide. Fig. 38 shows a
pulley 28^ in. x 17 in. suitable for this machine. The pulley
is provided with one set of arms only, as the face is not very
wide. Setscrews are provided to prevent the pulley work-
ing endwise on the shaft.

COITN'ECTIONS

80. The electrical connections for alternators have
already been shown diagrammatically; it is now necessary
to see how these are carried out on the machine. We will
first consider the connections suitable for a single-phase
compound-wound machine of the type designed. Fig. 39
represents the connections of such a machine. T^and T'
are the two terminals of the armature winding, one of which
is connected to one collector ring by means of the stud a.
The other terminal T' is connected to one side of the recti-
fier by the stud d, the other side of the rectifier being con-
nected to the remaining collector ring. If a revolving
shunt is used across the rectifier, it is necessary to have
another connection stud, shown by the dotted line. The
revolving shunt is then connected between this stud and d,
thus placing the shunt across the rectifier and allowing a
certain portion of the total current to flow by without being
rectified. The line wires lead from the two collector rings,
and the rectifier brushes are connected to the series-field by
means of the connection boards c, c. The connections
between the series-field, armature, rectifier, and collector
rings shown in Fig. 39 are those that are used on the
General Electric Company's machines of this type. The
Westinghouse Company uses a different arrangement for
supplying the rectified current to the series-coils, which is

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DESIGN OF ALTERNATING

§21

shown in Fig. 40. In this case the terminal T is connected
to one end b of the primary a b oi a, small transformer.
The other end of this primary connects to the collector
ring, as shown, so that all the current flowing through the
armature passes through this coil. The secondary ^^ of
this transformer connects directly to the two sides of ^he
rectifier, which, in turn, connects to the series-field by

-•VNAAAAAA^

Pig. 89

means of the brushes. The other collector ring is con-
nected directly to the winding, as shown. In this case it is
seen at once that there is no electrical connection between
the armature and the series-coils, the latter being supplied
by an induced current from the secondary c d. This trans-
former, which is usually quite small, must, of course,
revolve with the armature, and in some of the smaller

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§21

CURRENT APPARATUS

machines the spokes of the spider form the core of the
transformer. The use of this transformer renders the insu-
lation of the series-coils easier, because it separates the
armature connections entirely from the field.

Fio. 40

81, -The connections for the field coils vary little in
different makes of machines, so we will take those shown in
Fig. 39 as a typical case. The windings of the field coils
are connected up so as to make the poles alternately N and S.
Care must be taken that the series-coils are not connected
in such a way as to oppose the separately excited coils
instead of aiding them. The terminals of the separately
excited coils are led directly to the connection boards r, c.
The terminals of the series-coils are also led to the same
boards, and from there connected to the rectifier brush-
holder studs by means of flexible cables. The stationary

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60 DESIGN OF ALTERNATING §21

shunt d IS connected to the same terminals on the connec-
tion boards as the series-field. This shunt may be attached
to the machine or placed on the switchboard ; it is usually
made up of German-silver wire or ribbon of such size that
it will not overheat with the maximum current it may be
called on to carry. The connections and winding of the
separately excited coils are generally the same, no matter
what the current output or voltage of the machine may be.
The series-connections may, however, be varied somewhat
in machines with different current outputs. When the cur-
rent output is large, the series-coils are sometimes grouped
in two sets connected in parallel, thus reducing the cur-
rent in the field conductor and allowing the use of smaller
and more easily wound wire. For example, the 100-kilo-
watt machine designed had a full-load current output of
45.4 amperes at 2,200 volts; if the same machine were built
for 1,100 volts, the current output would be 90.8 amperes at
fuU load. In the first case the series-field was designed to
carry 32 amperes ; in the second case it would have to carry
64 amperes. Generally, we would wish to get the same num-
ber of ampere-turns on each pole in either case ; so, instead
of winding the coils with half as many turns of wire, large
enough to carry double the current, we can connect the six
upper coils in series and connect them in parallel with the
six lower coils, which are also connected in series. This will
keep the current in the coils the same, although the line
current is doubled. This is often done in practice, as it
allows the coils that were designed for a machine of certain
voltage to be used for a machine of half that voltage
without changing the coil winding in any way.

83, The line connections are usually made directly to
the collector-ring studs when the machine is provided with
a revolving armature. When the armature is stationary,
the armature terminals are simply run to a connection
board, to which the lines are attached. Fig. 41 shows a
simple form of connection board, suitable for the connec-
tions shown in Fig. 39. The base a should have high

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§21

CURRENT APPARATUS

insulating properties, and is preferably made of porcelain,
or hardwood treated with oil. Slate is not a good material
for this purpose, because it is liable to contain metallic
veins. Cable terminals c are provided for the connections,
and these are held in place by screws d passing through

PIO. 41

from the back of the base. These screws are well counter,
sunk, and the holes filled in with insulating compound, in
order to obviate any danger of the connections becoming
grounded on the frame of the machine. The nuts e clamp
the terminals firmly in place against the brass blocks b,

83. Connections between the individual field coils are
usually made by means of small brass connectors similar to
those shown in Fig. 42. Three of the commoner forms are
here shown. They all consist of two brass plates ^, / pro-
vided with grooves to receive the ends of the coils, and
clamped together by screws, as shown. The ends of the
coils usually consist of heavily insulated wire brought out
from the winding. In some cases where the coils are wound
with copper strip, connection between the coils is made by
simply clamping the ends of the strip together between
brass washers.

84. Special reference has not been made to the design
of fields for two- and three-phase machines, because there
is very little difference between such fields and the one

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DESIGN OF ALTERNATING

§21

worked out for the single-phase machine. The only differ-
ence might be a slight change in the series-winding and the

jCX

rrr3

■o

(a)

■o

(b)

re J

Pig. 42

connections to the rectifier. The winding of the separately
excited coils would be the same, because the exciter voltage
would not be changed, and all three fields were assumed to
furnish the same magnetic flux.

FlO. 48

. 85. Fig. 43 shows an assembled compound-wound
machine with stationary field and revolving armature, such

Digitized by VjOOQIC

§21 CURRENT APPARATUS 63

as we have worked out. The lower half of the yoke is in this
case cast with the bed, and the yoke itself is provided with
flanges. The col-
lector-ring brushes
are here shown
mounted on a
stand a, and the rec-
tifier brushes are car-
ried on a rocker b
mounted on the
inside end of the
bearing. The* ar
rangement of cables,
connection boards,
etc., will be readily

seen by referring to ^

the figure. Fig. 44 ""^^^ '^"

shows a large alter- ^®' ^

nator designed to run at low speed. This machine is pro-
vided' with a stationary armature and revolving field, the
collector rings shown on the shaft being used to convey the
exciting current into the field coils.

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DESIGN OF ALTERNATING-
CURRENT APPARATUS

(PART 8)

tra:n^sformers

1, It has been shown that a certain amount of loss
always occurs in a transformer so long as its primary it;
connected to a source of E. M. F. ; this loss may be divided,
for convenience, in two parts, namely, iron losses and cop-
per losses. The iron losses are those that occur in the iron
core of the transformer, and are due to hysteresis and
eddy currents. They are practically constant for all loads,
because they are dependent on the magnetic density in the
core, and this changes but little from no load to full load.
The I*R loss, or copper loss, in the coils increases with
the load. The combined effect of these losses is to heat
up the coils and core, so that the amount of power that a
transformer is capable of delivering is limited by the heat-
ing effect. The transformer could therefore be loaded until
the coils reached the maximum temperature that the insu-
lation on the wire could stand without injury; any further
increase in load would result in the transformer being
eventually burned out. Aside from the danger of over-
heating, a transformer should not be worked much beyond
Its rated load, because of the falling off in efficiency. If the
load is forced too high, the P R loss becomes excessive, and

For notice of copyright, see page immediately following the title page.
4^—9

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2 DESIGN OF ALTERNATING §22

the transformer works uneconomically, even if it does not
happen to overheat.

Overloading a transformer also causes a falling off in the
secondary voltage, which is very objectionable if the trans-
former is used for lighting work.

2. A transformer should be so designed that it will do
the work of transforming the current with the least possible
cost. This means that the efficiency must not only be high
at full load, but that it should also be high throughout a

T^rantfurmer effiei&ncy cwrve.

FlO. 1

wide range of load. Fig. 1 shows the efficiency curve for a
transformer of good design. It will be noticed that the
efficiency increases very rapidly at first, being as high as
60 per cent, with only one-sixteenth of the full load on the
secondary. The efficiency varies but slightly between one-
fourth load and full load, and when the transformer is over-
loaded, the efficiency begins to fall off. A transformer is
seldom worked at its full capacity all the time; hence, it is
important to have a good efficiency through a wide range
of load, as shown by the curve. The efficiency can be made

Digitized by VjOOQIC

§22 CURRENT APPARATUS 3

high by employing anything that will keep down the losses;
but for a transformer of given size, the efficiency cannot be
increased beyond a certain point without greatly increasing
the weight and cost. For example, the /" R loss might be
made very small by using a large cross-section of copper,
but this would necessitate a large winding space, thus
increasing the bulk of the transformer and making the core
heavy. Increasing the efficiency beyond a certain point is
attained only by a large increase in cost, and a transformer
may, in general, be said to be well designed when it gives
the highest all-day efficiency consistent with an economical
distribution of iron and copper. The curve. Fig. 2, shows
the relation between output and full-load efficiency that
should be attainable in good transformers. The efficiency

IMatUm beivnen effieUney and mUfut of trang/ormerB.

FIG 2

increases rapidly with the output for transformers of small
size, but changes slowly after outputs of 4 or 5 kilowatts
are reached. Some very large transformers have an effi-
ciency as high as 98 per cent., or slightly over, but it is
only in transformers of large size that such a high efficiency
is reached.

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DESIGN OF ALTERNATING 8 22

TRANSFORMER CORES

3. Transformer cores have been made in a large num-
ber of different shapes, but the two most generally used
types are the core and shell varieties. Good transformers
may be designed using either the core or shell construction,
and large numbers of both styles are in common use.
Great care should be taken in the selection of the iron for
transformer cores. It should be borne in mind that the
hysteresis loss goes on continuously, whether the trans-
former is loaded or not, and that everything possible should
be done to keep this loss small by using only the best qual-
ity of core iron. The stampings should be about 12 or
14 mils thick for 125-cycle transformers, but may be slightly
thicker than this for transformers of low frequency. The
oxide on the iron, with the addition of a paper sheet at
intervals along the core, is usually sufficient to insulate the
sheets from each other. Some makers coat the plates with
an insulating varnish or japan and do not depend on the
oxide film.

HEATING OF TRANSFORMERS

4. Since the efficiency of transformers is generally high,
the energy lost in them is small, and in transformers of
ordinary size there is generally enough radiating surface
to get rid of the heat generated. Transformers up to
50-kilowatt capacity can usually be made with sufficient
ventilation to get rid of the heat generated, but for larger
sizes it is often necessary to use special cooling arrange-
ments. Air blasts are frequently used to carry the heat
away from the core and windings of large transformers.
Sometimes the core and windings are immersed in oil kept
cool by water circulating in pipes. Transformers of smaller
size are usually designed so that the case may be filled with
oil. This helps to give the windings good insulation, and
keeps down the temperature by conducting the heat from
the windings and core to the outside casing. The student

Digitized by VjOOQIC

§22 CURRENT APPARATUS 5

should bear in mind that while these special devices are in
many cases necessary to get rid of the heat, it does not
follow by any means that the transformer is inefficient; on
the contrary, the efficiency is usually very high, and these
devices are necessary only because the transformer of itself
does not present enough radiating surface to get rid of the
heat. No definite, rules can be given as regards the number
of watts that can be radiated per square inch of core or case
surface that will apply to all types of transformers. This
radiation constant varies widely for transformers of different
sizes and forms, but unless the efficiency is very low, the
dimensions of transformers under 40 or 50 kilowatts are
usually such that they can get rid of the heat generated
without undue rise in temperature.

MAGNETIC DENSITY IN CORE

6. Transformer cores are worked at low magnetic densi-
ties in order to keep down the core losses and magnetizing
current. The hysteresis loss is proportional to the frequency,
and the eddy-current loss to the square of the frequency;
hence, for an allowable amount of core loss it follows that
higher magnetic densities can be used with low-frequency
than with high-frequency transformers. For 60-cycle trans-
formers, the maximum value of the magnetic density may
be taken from 28,000 to 32,000 lines per square inch. For
125-cycle transformers, the density may be from 19,000
to 21,000 lines per square inch. The densities in individual
cases may vary from the above, but the average values used
are generally within the limits given.

6. The allowance of copper per ampere in the primary
and secondary coils should be large, in order to keep down
the copper loss and prevent overheating. The coils are
usually heavy, and it is also important to have a liberal
cross-section of copper, in order to prevent overheating.
The cross-section per ampere should be about the same both
for primary and secondary coils. When the core type is

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6 DESIGN OF ALTERNATING §22

used, there is usually room for a liberal cross-section of
copper, but in the shell type the winding space is more
restricted, and the coils cannot be made very large without
considerably increasing the bulk of the iron core. The
number of circular mils allowed per ampere varies greatly
in transformers of different makes and sizes. In general,
the allowance should not be less than 1,000 or 1,200 circular
mils per ampere, and in many of the later types of trans-
formers the allowance may be as high as 2,000, or over.

ARRANGEMENT OF COIIiS AND CORE

7. The arrangrement of colls and core has already
been described for two of the common types. The core type
can be usually arranged so that it can be taken apart and
the coils slipped off in case repairs are necessary, while the
shell construction usually requires the removal of each plate
before the coils can be reached. Transformers have been
made with the core built in sections, as shown in Fig. 3. In
this case the upper part^ is built up separately, and forms
a cover that can be removed from the main part of the core
when it is desired to get at the coils. This construction is,
however, objectionable, because it introduces small air gaps
into the magnetic circuit at ^, b, thereby increasing the mag-
netic reluctance. In designing transformer cores, every effort
should be made to have the magnetic circuit continuous.
Fig. 4 shows an arrangement of coils and core suitable for a
transformer of large size. The stampings a and b are cut
as shown, the joints being at r, d, and e. As the core is
piled up, these joints are staggered, as shown by the dotted
lines, thus making the iron path for the lines practically
continuous and doing away largely with the bad effects of
the joints. The primary and secondary coils are wound in
a number of sections, each consisting of a flat coil, these
sections being sandwiched, as shown, in order to reduce the
magnetic leakage between them. Splitting up the coils in
this way also makes it easier to insulate the transformer for

Digitized by VjOOQIC

CURRENT APPARATUS

high voltages, because it cuts down the voltage across any
one of the coils. The coils are usually separated from each

,'"

i%tifSftl%*^J^%%%*i^i

v^-d

'^^

W^^^

^^™
^

ft.

ft«

w
"^T

)

Other by a built-up sheet of mica, or other material having
high insulating properties. Large cores are frequently

Digitized by VjOOQIC

DESIGN OF ALTERNATING

§22

provided with ventilating ducts between the laminations, as
shown aty. The laminations are held apart by brass cast-
ings, and the channels so formed allow air to circulate
through the core, the whole construction being similar to
that used for ventilated armature cores. Fig. 5 shows
another arrangement of coils and core that also makes use
of thin flat coils. In this case the stampings a and b sur-
round one side of the coil only, a separate set of stampings

rrr

B i B

FlO. 5

being used to form the magnetic circuit around the other
side. This is the construction used by the Westinghouse ^
Company for several of their larger transformers. /The
projecting ends of the coils c are frequently spread out
like a fan, so as to allow air to circulate freely between
them.

WINDING AND INSULATION OF COILS

8, Since transformer coils are usually of simple shape,
they can generally be lathe-wound and thoroughly insulated.
High insulation is of great importance in transformers, and
every precaution should be taken to see that the primary
and secondary coils are not only well insulated from the core,
but also from one another. Fig. G shows the shape of a
primary coil commonly used for shell transformers. The
coils must withstand a high impressed line E. M. F., and
the voltage between layers may therefore be considerable

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CURRENT APPARATUS

Insulation i should be placed between each layer ; this may
be composed of oiled linen tape or other good insulating
material. The outside of the coil is heavily taped and after-
wards treated with insulating varnish and baked. Addi-
tional insulation in the form of mica and paper, or in some
cases oiled hard-wood pieces, is placed between the coils and
the core. The insulation between primary and secondary
should be specially good. Some makers allow a clear air
space between the coils, in addition to the insulation on

FlO. 6

the coils themselves. If connection should be established
between the primary and secondary, and there should hap-
pen to be a ground on the primary mains, a difference of
potential would exist between the secondary service wires
and the ground that would be equal to the primary voltage.
Such a difference of potential between the service wires and
the ground would be very dangerous to life; hence, the
importance of thorough insulation between the primary
and secondary.

9. The conductor used for the primary winding usually
consists of copper wire, except in large transformers, where
copper strip may be used to advantage. For the secondary,
a conductor of large cross-section is usually required,
because the secondary voltage is generally low and the cur-
rent correspondingly large. For transformers of moderate
output, the secondary conductor can generally be made of a
number of wires in multiple. In most large transformers,
the secondary conductor is made up of copper strip. Fig. 7
shows a flat secondary coil made up in this way. Such a

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DESIGN OF ALTERNATING

coil would be suitable for the transformer shown in Fig. 5.
The details of construction and method of calculating the

PIO. 7

different parts will be best understood by working out an
example. We will therefore take up the design of a trans-
former of the core type such as would be suitable for
lighting work.

DESIGN OF 8-KIIiOWATT TRANSFORMER

10. In starting out to design a transformer, the follow-
ing quantities are either known or assumed : Useful second-
ary output in kilowatts (K. W.); primary voltage {£p);
secondary voltage {£,) ; frequency of system on which the
transformer is to be operated (n). For ordinary lighting
transformers, Ep is in the neighborhood of 1,000 or
2,000 volts; £„ 50 or 100 volts; and ;/, 60 or 125 cycles per
second.

11, We will take for an example an 8-kilowatt trans-
former of the core type to be designed for 2,000 volts pri-
mary and 50 or 100 volts secondary, the secondary being
wound in two coils, which may be connected in parallel for
50 volts or in series for 100 volts. The frequency will be
taken as 60. A good transformer of this output should
have a full-load efficiency of 96.8 or 96.9 per cent.; conse
quently, in designing it we should aim to keep the losses

Digitized by VjOOQIC

§22 CURRENT APPARATUS 11

down to such an amount that the efficiency will be, say,
96.8 per cent. We have

^ . watts output

efficiency = ; — ^-—

-' watts input

Hence, for an output of 8,000 watts, the input will be

input = -^^ = 8,264 watts

The total loss at full load, therefore, should not exceed
264 watts. This total loss is made up of three parts,
namely, the losses due to the resistance of the coils, hyster-
esis, and eddy currents. The P R loss and the core losses
should be about equally divided; that is, the copper loss
should be about equal to the sum of the hysteresis and eddy-
current losses. If the transformer is used only a short time
during each day, it might be well to allow the /" R loss
to be a little larger than the core losses, but the above
relation holds approximately correct for well-designed trans-
formers. In the present case, we will aim at making the
copper loss, say, 140 watts, and the core loss 124 watts.
This division of the losses should give a satisfactory trans-
former for lighting work.

BETEBMINATIOX OP CORE VOIiTTMK

12. Since the transformer is to operate on a 60-cycle
system, we will take 30,000 lines per square inch as a fair
value for the maximum magnetic density in the core. At
this frequency and density, there will be a definite amount
of loss per cubic inch of iron in the core, depending on the
quality of the iron used. We will assume that the curve A^
Fig. 2, Part 1, represents the quality of the iron in this
respect. From this curve, we find that the loss per cubic
inch per 100 cycles at a density of 30,000 is about .25 watt.
The loss at 60 cycles will therefore be ^^ x .25 = .15 watt.

The total core loss is to be 124 watts. This is the loss
due to hysteresis and eddy currents combined. The eddy-
current loss should be quite small if the core is properly

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DESIGN OF ALTERNATING

§22

laminated ; hence, we will take the hysteresis loss alone as
110 watts, and allow 14 watts for the loss due to eddy
currents. If the loss per cubic inch is .15 watt, then the

volume of iron in the core will be — r^ = 733 cubic inches.

.15

DIMENSIONS OF CORE

13. The volume of iron in the core has now been deter-
mined, and it remains to proportion the core itself. Fig. 8
shows the style of core used for this type
of transformer, and in proportioning it
due regard must be had to the windings
that are to be placed on the cores c, c.
We will make the core square in cross-
section, with the corners chamfered
slightly, as shown in the figure. If the
cross-section ^ x ^ is made very small,
the cores will be long and thin, the mag-
netic flux ^ will be small, and the coils
will have to be provided with a large
number of turns to generate the required
E. M. F. Long cores also give rise to a
long magnetic circuit, thus increasing
the magnetizing current. On the other
hand, if the cores are made very short,
the wire will have to be piled up deep, in order to get it into
the winding space, and the yokes across the ends will have
to be made longer. Deep windings also mean a long length
of wire for a given number of turns, resulting in a large
amount of copper. The best proportions to be given to the
core are therefore largely a matter of experience. For pre-
liminary dimensions, we will use the proportions indicated
in Fig. 8, all the dimensions being here expressed in terms
of the thickness of the cores. We will make the height of
the core = 1 a. The volume of the core will then be

3.5 a •

<»

^-H

^IJ^a* mo-

«0

f»

<»

Fig. 8

V = {%x^.6a^\-%Xba)a'

(1)

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CURRENT APPARATUS

a* being the area of cross-section and 5 a the distance
between the yokes. This ^

gives r- '*ir^

na' = V= 733 cubic inches

\3f

^(^

■?r-

a =

This makes a just about
3J inches. This is the value
of the thickness of the core
if it were solid iron. Part
of the cross-section is, how-
ever, taken up by insulation
between the plates, and the
corners are cut off slightly,
so we will make the core
3f inches square. The
other dimensions follow
from this, so we will take
the dimensions given in
Fig. 9 as a basis for further
working out the design.
The distance between the
inside edges of the cores will be 5^ inches, and the
space between the yokes available for the windings will
be 18| inches.

'vl.

PIO. 9

DIMENSIONS OF CONDUCTORS

14, We will wind the secondary coil next the cores, and
place the primary over it. The secondary current at full
load will be

secondary watts 8,000 .^ ,o\

:i T7- = "TTTTT- = 8^ amperes (2)

secondary volts 100 ^ ^ '

The secondary coil will be wound in two sections, one
section being placed on each core. Each section will have
a sufficient number of turns to generate 50 volts, and the

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U DESIGN OF ALTERNATING §22

conductor will be capable of carrying 80 amperes. If an
output of 100 volts and 80 amperes is required, the coils
may be. connected in series and their E. M. F.*s added. If
an output of 160 amperes at 50 volts is desired, the coils
may be connected in parallel. In either case, the full-load
current in the conductor will be 80 amperes. In this type
of transformer, a large cross-section is usually allowed per
ampere, because there is plenty of room for the coils, and
the number of turns is usually large. We will therefore
allow 2,000 circular mils per ampere to obtain the approxi-
mate size of the conductor. We then have

Circular mils cross-section of secondary conductor = 80
X 2,000 = 160,000 circular mils.

Six No. 6 B. & S. wires in parallel will give 6 X 26,250
= 157,500 circular mils. We will make up the secondary
conductor, as shown in Fig. 10, using six bare
wires in multiple and covering the whole with
a cotton insulation having a double thickness
of 20 mils. The bare diameter of the wire is
.162 inch; hence, the width of the conductor
over all will be 2 X .162+ .02 = .344 inch.
The height of the conductor will be 3 X .162 + .02
= .506 inch.

16, The watts supplied to the primary at full load are
8 264. Hence, the approximate primary current will be

primary watts 8,264 . ,^^, .«.

' u- = TTT^KK = ^-132 amperes (3)

primary volts 2,000 ^ ^ '

The primary current at full load will be very nearly in
phase with the E. M. F. ; or, in other words, the power
factor will be very nearly 1. The magnetizing current
should be quite small, so that the primary current at full
load will be but slightly larger than the above amount.
We will call the full-load primary current 4.25 amperes, in
order to allow a little for the magnetizing current. Allow-
ing the same cross-section per ampere in the primary as
in the secondary, we get

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§22 CURRENT APPARATUS 15

Circular mils of primary conductor = 4.25 X 2,000 = 8,500
circular mils.

A No. 11 B. & S. wire has a cross-section of 8,234 circular
mils, which is nearly the number required. The diameter
of this wire over the insulation may be taken as .101 inch.

CALCULATIOX OF PRIMARY AND SECONDARY TURNS

16. The primary coil has to be provided with a sufficient
number of turns to generate a counter E. M. F. equal and
opposite to that of the mains. The impressed E. M. F.
is equal and opposite to the resultant of the E. M. F.
generated by the primary and the E. M. F. necessary to
overcome the resistance of the primary. The drop through
the primary at no load due to the ohmic resistance is so
small that it may be neglected in comparison with the
E. M. F. that is generated by the primary coil, so that we
may take the E. M. F. so generated as equal numerically to
the impressed E. M. F. The number of turns required to
set up this E. M. F. will depend on the magnetic flux 0
that threads through the turns. The maximum magnetic
flux through the coil will be

^ = B max. X A (4)

where B max. is the maxfmum value that the magnetic den-
sity reaches during a cycle, and A is the area of cross-section
of iron in the core on which the coil is wound.

In this case, B max. is 30,000 lines per square inch, and
the area of cross-section of the iron is3i x 3^ = 12.25 square
inches; hence,

0 = 30,000 X 12.25 = 367,500 lines

Taking the E. M. F. generated in the primary as the
equal and opposite of the line voltage, we may write

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16 DESIGN OF ALTERNATING §22

where ^ = maximum value of the magnetic flux through
the core ;
7^ = number of turns on primary coil;
;/ = frequency (cycles per second) ;
Ej, = impressed primary voltage.
Applying this to the present example, we have

2,000 = k^^^il^im2iJ).>i^

„ 2,000 X 10" - . .^- ,

^^ = Or3r367,500 X 60 = ^'«*^' "^^'"'y

We will therefore provide the primary coil with, say,
2,040 turns, and place 1,020 on each of the cores, as this
number will give an even number of turns on each coil.
Dropping two turns would not appreciably affect the work-
ing of the transformer, as the magnetic density would have
to be increased but very slightly to make up for the dif-
ference.

17, The number of secondary turns T, will be

T,X§- (6)

where E^ is the secondary voltage, since the turns must be
in the same ratio as the voltages generated. This will give
for the present case

The total number of secondary turns will therefore be 102,
or there will be 51 turns on each coil, using the conductor
shown in Fig. 10.

ARRANGEMENT OF PRIMARY AND SECONDARY COILS

18, The coils will be arranged on the core as shown in
the section through the coils and core, Fig. 11. The coils
are here shown circular in cross-section; very often they are
approximately rectangular in shape, the secondary being

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§22 CURRENT APPARATUS 17

wound directly on the core and the wooden pieces a, a
omitted. In the larger sizes of transformer of this type,

cylindrical coils are

commonly used.
The secondary will
be wound next the
core, in order to
make the length of
the heavy second-
ary conductor a s
short as possible.
The coil may be
held firmly in posi-
tion by oiled hard-
wood blocks a
placed between it
and the iron core b.

The diameter of the coils could be made somewhat less by
chamfering the comers more than shown, but this would
decrease the cross-section of iron, so that very little would
be gained in the end. Both coils are heavily insulated with
linen tape, and provision is made for a clear space of ^ inch
between the primary and secoAdary. The length of the cores
between the yokes is 18| inches (see Fig. 9). Each second-
ary coil contains 51 turns. The breadth of each turn is
.344 inch, so that 51 turns will take up a length along the
core of 61 X .344 = 17.5 inches. The secondary coil can
therefore be made up of one layer of 51 turns of the con-
ductor shown in Fig. 10. This arrangement will allow
about -^ inch clearance at each end between the secondary
winding and the yoke, in addition to the taping. The
arrangement of this winding will be readily understood by
referring to the section of the coils shown in Fig. 13. The
mean diameter of the secondary coil will be 5^ inches and
the mean length of a turn 17.28 inches.

19. The primary coil is placed over the secondary, as
shown in Fig. 11. The space of ^ inch is allowed to insure

45—10

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18 DESIGN OF ALTERNATING §22

good insulation between the coils ; sometimes a mica insulating
shield is placed between the coils. In case the transformer is
immersed in oil, the film of oil between the coils forms an
insulating layer that is not easily broken down. We will
make the primary coil slightly shorter than the secondary,
and adopt a clear winding space, say, 17^ inches in length.
This will remove the high-tension primary windings a little
farther from the yokes and avoid danger of breakdown.
The diameter of the primary conductor over the insulation
is .101 inch; hence, in a layer 17^ inches long we can place

' = 170 turns, nearly. We must place 1,020 turns on

each coil, so that we can arrange the winding by using
six layers of 170 turns per layer. The two primary coils
are connected in series across 2,000- volt mains; hence, the
pressure across each coil will be 1,000 volts, and there will be
166 volts generated in each layer. The pressure tending to
break down the insulation between the beginning of the
first layer and the end of the second will therefore be the
maximum value corresponding to an effective pressure of
333 volts. It is necessary, therefore, to insulate each layer
from the one next to it, and particular care should be taken
at the ends of the coil, where a breakdown between layers is
most liable to occur. We will allow 20 mils for insulating
tape between each layer and j\ inch all around for the outer
taping on the coil. This will make the thickness of the
primary coil 6 X .101 + 5 X .020 + J = .831 inch.

The mean diameter of the primary coil will be about
7| inches, and the mean length of a primary turn 23.17 inches.

30. All the essential dimensions of the transformer have
now been determined. With the primary winding calcu-
lated above, the outside diameter of the primary coil will
be about 8J inches. The distance from center to center of
cores is 6^\ -f 3| = 9 ^^ inches, so that there would be a
space between the coils of \^ inch, and the design is suitable
^s far as the accommodation of the windings goes.

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§22 CURRENT APPARATUS 19

EFFICTEKCY

21. In designing the transformer, we aimed at securing
a certain eflttciency, and so proportioned the core that the
hysteresis loss should not exceed 110 watts. The design
has been worked out, and it is found that the windings
obtained can be accommodated on this core. It now
remains to be seen whether the copper loss in these coils is
within the allowable amount. If the copper loss is exces-
sive, we must remodel the design of the coils so as to bring
it to nearly the allowable amount. In order to calculate the
copper losses in the primary and secondary, we must first
determine their resistance.

22. In calculating the resistance of the coils, we will
take the resistance of a mil foot of copper as 12 ohms, as it
is the hot resistance that we must consider. Since there
are 51 turns on each secondary coil, and the length of each
turn is 17.28 inches, the resistance of each coil will be

total length in inches 51 X 17.28 ^^^^ ,

R^ = 1 — -. i TT- = .^^ ^^, = .0056 ohm

^ area m circular mils 167,506

and the resistance of the two coils in series will be .0112 ohm.
The loss in the secondary at full load will therefore be

//i?. = (80)* X .0112 = 71.68, say, 72 watts (7)

23. Each primary coil has 1,020 turns, and the length
of each turn is 23.17 inches. The resistance of each pri-
mary coil will then be

„ 1,020 X 23.17 o o»v u
^- = 8,234 = ^'^^ ^^"^^

and the resistance of the two coils in series will be 6.74 ohms.
The primary /* R loss will therefore be

//i?^ = (4.25)' X 6.74 ^ 103.7 watt$ (8)

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20 DESIGN OF ALTERNATING §22

The total I^ R loss in the coils as designed is about
176 watts instead of the 140 watts allowed. The difference,
however, is not great enough to make a very large differ-
ence in the efficiency. It will be noticed that the less in
the primary coils is rather high, since the loss should be
about equally divided between the primary and secondary.
This can be remedied to some extent by lowering the pri-
mary resistance, i. e., by using a larger wire for the primary
winding. We will have room enough to do this because
we found that there would be a clearance of W inch
between the coils. Suppose we try a No. 10 wire for the
u Tiary and see if this will give a more satisfactory result,
nsulated diameter of this wire will be about .112 inch.
The number of turns that can be placed in one layer will be

17 25

' = 154. We will therefore use six complete layers with
.ll/«

154 turns each, and one additional layer with 96 turns. The
coil at the part where it is wound seven layers deep will
have a thickness of 7 X .112 + 6 X .020 + ^ = 1.029 inches.
This will increase the mean diameter slightly and make
the mean length of a turn about 23.3 inches. The cross-
section of the wire will now be 10,380 circular mils, so that
the resistance of each primary coil will be

^ 1,020X23.3 o oo u
^= 10,380" - = ^-^^^^^'

and the resistance of the two coils will be 4.58 ohms.
The loss in the primary at full load will then be

Ip^Rp = (4.25)' X 4.58 = 83 watts, nearly

This makes the total /- R loss 72 + 83 = 155 watts,
instead of 176. This change in the primary winding makes
the loss in the primary and secondary more nearly equal,
and brings the total loss down nearly to the required
amount. We will therefore adopt this winding in place of
the one previously calculated. The outside diameter of the
primary coils will now be a little over 8J inches, so that
there will still be a clearance of about \ inch between them

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CURRENT APPARATUS

when the transformer is assembled. The total loss at full
load will be 110 + 155 + 14 = 279 watts. The full-load
efficiency will then be |J^g = .9663, or about .17 per cent,
lower than was assumed when starting out to design the
transformer.

EFFICIENCY CURVE

34. The curve showing the relation between the effi-
ciency and output can be readily plotted when the efficiencies
at different loads are known. We will therefore calculate
the efficiency for one-eighth, one-fourth, one-half, three-
fourths, and full load, also for one-fourth overload. In
order to do this, we will assume that the core loss remains
constant. For example, at one-fourth load the useful out-
put is 2,000 watts, and the secondary current 20 amperes.

TABX.E I

Secondary
Output

Watts

Secondary
Current
Amperes

Primary Current
Amperes
(Approx.)

0
0

iil
t

me*

I.OOO

.60

124

1.65

1. 12

126.8

1,127

88.73

a.ooo

1. 12

124

5.72

4.48

134.2

2,134

93.72

4,ooo

2.16

124

21.39

17.92

163.3

4,163

96.08

6,ooo

3- 20

124

46.9

40.32

211. 2

6,211

96.60

Full load

8.000

4.25

124

83.00

72.00

2790

8,279

96.63

J overload

lo.ooo

lOO

5.30

124

128.65

112.00

365.0

10,365

96.48

The primary current will be that corresponding to the sec-
ondary current of 20 amperes (or 1 ampere, since the ratio
of transformation is 1 to 20) plus the current necessary to set
up the magnetization and supply the losses. The primary
current at one-fourth load may be taken as approximately
1.12 amperes, since the amount of current required to
supply the losses will be very small at this load. The

Digitized by VjOOQIC

22 DESIGN OP ALTERNATING §22

primary PR loss will be (1.12)' X 4.58 = 5.72 watts. The
secondary I* R Iqss will be (20)' X .0112 = 4.48 watts.
The core loss is 124 watts; hence, the total loss will be
134.2 watts. The input will then be 2,134 watts approxi-
mately, and the output 2,000, giving an efficiency at this
load of 93.72 per cent. The calculations and results for the
other loads are given in Table I.

36, These values of the load and efficiency give the
curve shown in Fig. 12. The student should compare this

§>

Output in fraction* of full load.
Efficiency curve ftnr transformer desiffned.

PIO. 19

curve with that shown in Fig. 1. The scale used for the
efficiency in Fig. 12 is larger than that in Fig. 1, in order to
show the variation of efficiency more clearly, but it will be
noticed that the curves have the same general character-
istics. The variation in efficiency in this case is not more
than 3 per cent, from one-fourth load to 25 per cent, over-
load. It will be seen from the table that the efficiency
begins to drop off when the transformer is overloaded,
owing to the rapid increase of the /* R losses.

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22 CURRENT APPARATUS 88

AMJ-DAY EFFICIENCY

26. The eflSciency that actually determines the cost of
operating the transformer is the all-day efficiency, or the
ratio of the watts useful output per day to the watts sup-
plied during the day. This will depend on the length of
time during the day that the transformer is doing useful
work. For example, suppose the transformer were worked
during the 24 hours an amount equivalent to full load for
6 hours, and that it remained idle an amount of time equiva-
lent to 18 hours. The core losses would go on for the whole
24 hours, because the pressure is maintained across the lines,
whether the transformer is working or not. The watt-
hours wasted in the form of core losses in 1 day would
therefore be 124 x 24 = 2,976. The copper losses during
1 day would be equivalent to the sum of the primary and
secondary full-load copper losses for 6 hours. Hence, the
watt-hours energy wasted in PR losses per day will be
155 X 6 = 930. The useful energy delivered during the
day is equivalent to full load for 6 hours, or 8,000 X 6
= 48,000 watt-hours. The energy that must be supplied
during the day is 48,000 -f 2,976 + 930 = 51,906 watt-hours,
and the all-day efficiency under these conditions is jf ^g
= .925, nearly. This means that of all the energy delivered
to the transformer during 24 hours, 92.5 per cent, is con-
verted into useful energy and the remainder wasted. If the
transformer were loaded for a longer period during the day,
the useful work done would be greater and the /* R loss
would also be greater. The core loss would remain the
same as before, so that the all day efficiency would depend
on the relation between the copper and iron losses. . For
example, suppose the transformer were fully loaded for
10 hours instead of 6. The useful work would be 80,000 watt-
hours and the energy wasted in copper losses 1,550 watt-
hours. The core loss would be 2,976, as before, and the
total energy supplied would be 84,526, giving an all-day effi-
ciency of about 94.6 per cent. The condition of load for
which any given transformer will give its maximum all-day

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U DESIGN OP ALTERNATING §22

efficiency depends, therefore, on the relation between the
copper and iron losses. It also follows that if the trans-
former is to be loaded for only a short period during the
day, the iron losses should be small if the all-day efficiency
is to be high.

MAGNETIZING CURRENT

37. The current that the primary of a transformer takes
from the line when its secondary is an open circuit is usually
spoken of as the lua^netizingr current, although, strictly
speaking, it is the resultant of the magnetizing current
proper and the current that represents the energy necessary
to supply the core losses. It is important that this no-load
current should be small, because if a large number of trans-
formers are connected to the line, the sum of all the mag-
netizing currents required by the separate transformers may
represent a considerable current to be supplied from the
station. This means that the alternator may be delivering
a fairly large current when no useful work is being done.
It is true that this current may not represent very much
powei, because it is considerably out of phase with the
E. M. F., but it loads up the lines and alternator, and thus
limits their useful current-carrying capacity. The no-load
current is made up of two components, one of which is the
magnetizing current, or the current that sets up the ampere-
turns necessary to drive the flux around the core. The
other component represents that current which is neces-
sary to supply the core losses, and is in phase with the
impressed E. M. F. The core loss in this case is 124 watts;
hence, this component of the no-load current will be ^^^
= .062 ampere.

38. The component of the no-load current that repre-
sents the current necessary to set up the magnetic flux may
be obtained as follows : For a magnetic density of 30,000 lines
per square inch, we will require about 5.5 ampere-turns per
inch length of the circuit for a good quality of transformer

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§22 CURRENT APPARATUS 26

iron. The mean path for the magnetic flux is shown by the
dotted line, Fig. 9; the length of this circuit is about
60 inches. The ampere-turns required to set up the flux
will then be 60 X 5.5 = 330. The number of primary turns
surrounding the circuit is 2,040. We then have magnetizing
current X 2,040 = 330, or current = .162 ampere.

The no-load current is therefore made up of the two com-
ponents .062 and .162 at right angles to each other, and its
value is I^ = i^. 062' + .162' = .17 ampere.

This is the current that the transformer will take from
the line when it is operating under no load. This does not
mean, however, that it is consuming .17 X 2,000, or 340 watts,
because the no-load current is always considerably out of
phase with the E. M. F., and, as a matter of fact, the trans-
former consumes only sufficient power to make up for the
core losses and the slight loss in the primary due to the
no-load current. At no load, the power factor may be con-
siderably less than 1, but as the load is increased, the cur-
rent and E. M. F. shift into phase until the power factor at
full load is very nearly unity.

REGUIaATION

39. As the secondary voltage will fall off as the load is
applied, it is important that this falling off should be slight.
In well-designed transformers the falling off in secondary
voltage may vary from 1 to 2.5 or 3 per cent., depending
on the output. This drop is due to magnetic leakage and
the resistance of the primary and secondary coils. In the
type of transformer designed, the falling off due to mag-
netic leakage will be quite small, because the coils are
wound one over the other, making the path between the
coils, through which leakage is set up, long and of small
cross-section. The leakage drop would not likely amount
to more than .2 or .3 volt. The drop in the secondary coils
at full load will be current X resistance = 80 X .0112
= .89 volt. The drop in the primary at full load due to

Digitized by VjOOQIC

-LI-

^\%r

Ili =

^;=^

tnehmm. .

PlO. 18

Digitized by VjOOQIC

§22 CURRENT APPARATUS 2l

the primary resistance will be 4.25 X 4.58 = 19.46 volts.

This drop of 19.46 volts in the primary will cause a corre-

19 46
sponding drop of = .97 volt in the secondary. The

total drop due to leakage and resistance combined will
therefore be under 2 volts, or 2 per cent, of the output,
which is close enough regulation for all practical purposes.

CONSTRUCTION

30. The constrnction and arransrement of the core
and coils will be understood by referring to Fig. 13. This
shows an elevation of the assembled transformer, with a
longitudinal section of the coils showing the windings and
insulation. The core is built up out of thin iron strips,
which are interleaved at the corners, so as to practically do
away with joints in the magnetic circuit. The plates are
shown 'held in position by clamps ^, drawn up by bolts b.
The terminals of the coils should be very heavily insulated,
and may be run to a connection board within the transformer,
or taken directly out through the case. Transformers in
sizes up to 20 or 30 kilowatts are usually placed in an
iron case arranged for mounting on poles. These cases
should be weather-proof, and made as light as possible con-
sistent with the necessary strength. They are generally
designed with a view to being filled with oil. Fig. 14
shows a case suitable for the transformer, designed. This
is made of cast iron about iV ^^ i ^^^^ thick. The case a is
provided with a cover d, which is bolted on by means of the
bolts d. The overlapping flange and gasket c serve to make
the cover water-tight. The transformer, which is shown by
the dotted outline, is held in place by setscrews. The pri-
mary terminals are brought out through the bushings e, and
four bushings /are provided on the front of the case for the
secondary terminals. The bushings should be of heavy
hard rubber or porcelain, and so constructed that they will
prevent leakage of current from the lines to the case.

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DESIGN OF ALTERNATING

These outlets should, of course, be directed downwards, so
that the wires may be looped into them, thus preventing
water from getting into the case. Lugs g^ g should be pro-
vided on the back of the case for attaching suspension hooks.
Fuses are usually provided between the primary and the

PIO. 14

line, but these are generally mounted outside the trans-
former case in separate fuse boxes of special construction.
Secondary fuses are not provided at the transformer, the
fuses in connection with the secondary service wires being
depended on to protect the secondary circuit. For large
indoor transformers, only sufficient covering is used to

Digitized by VjOOQIC

CURRENT APPARATUS

protect the coils, a regular case being unnecessary, as well
as interfering with the ventilation.

31. The transformer that has been worked out is one
that would be used on an ordinary lighting circuit. The
method of designing a step-up transformer would be essen-
tially the same, except that extra precautions would be
taken to insure very high insulation, and a larger allowance
of winding space would be necessary. The design of a shell
transformer may be also carried out in about the same way.
The core proportions shown in Fig. 15 may be taken as a

PlO. 16

starting point. All dimensions are referred to the width of
the tongue a^ which carries the lines through the coils. The
length of the core may be from 3 to 7 times a. The height
of the winding space is usually from 2 to 3 times a, and the
breadth from .7 to .8 times a. The thickness of the outer
part of the shell around the coils is necessarily one-half of a,
because this part of the core carries one-half the flux pass-
ing through the coils. In this type of transformer the
allowance of copper will usually be somewhat less than in
the core type, because the winding space is more restricted.

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30 DESIGN OF ALTERNATING 8 22

INDUCTION MOTORS

32, In many respects the action of an induction motor
resembles that of a transformer, and, consequently, parts
of its design can be carried out by methods similar to those
used in designing transformers. The primary of the induc-
tion motor, that is, the part into which currents are led
from the line, corresponds to the primary coil of the trans-
former, while the secondary, or the part in which the cur-
rents are induced, corresponds to the secondary. This
relation holds, whether the primary or secondary is the
revolving part; but in all that follows we will consider the
primary as being fixed and the secondary as revolving. In
•such an arrangement, the fixed primary is commonly spoken
of as the field, or stator, and the secondary as the arma-
ture, or rotor. The essential difference between an induc-
tion motor and a transformer is that in the latter case the
secondary core and windings are fixed as regards the primary,
and the E. M. F. generated in the secondary is made use of to
supply useful electrical energy to an outside circuit; while in
the former case the Secondary core and windings revolve with
regard to the primary, and the mechanical torque action
between the primary and secondary is made use of to
deliver mechanical energy. The currents generated in the
secondary are not led into an outside circuit, but flow within
the secondary itself, in order that they may react on the
field produced by the primary and so cause the armature or
secondary to exert the required effort at the pulley. A
transformer supplied with a constant primary pressure will
furnish a nearly constant secondary pressure independently
of the load ; an induction motor when supplied with a con-
stant primary pressure will run at nearly constant speed
independently of the load.

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CURRENT APPARATUS 31

lilMITATION OF OUTPUT

33. The outpnt of Induction motors, like that of
alternations and transformers, is limited principally by the
heating effect due to the various losses that occur in the
motor when it is loaded. The principal loss is that due to
the resistance of the primary and secondary conductors
although the hysteresis and eddy-current losses may also be
considerable if the motor is not properly designed. If an
induction motor is considerably overloaded, the armature
currents react excessively on the field, causing excessive
magnetic leakage along the air gap, and greatly lessening
the torque between the field and armature. If the overload
is sufficiently great, the torque will be reduced to such an
extent that the motor will stop. Usually, however, an
induction motor may be loaded for short periods beyond
its full-load capacity without danger of overheating or.
stopping.

PRIMARY CORE LOSSES, MAGNETIC DENSITIES, ETC.

34. The losses in the primary are made up of the core
loss due to hysteresis and eddy currents, and the copper
loss due to the resistance of the primary winding. The
frequency of the changes in the magnetism of the primary
is the same as the frequency of the current magnetizing
it; hence, the lower the frequency at which the motor is
operated, the higher is the allowable value of the magnetic
density in the primary core. The core densities used for
such motors should be about the same as those used for
transformers operating at the same frequency. The curve,
Fig. 16, shows the relation between the maximum value of
the density and the frequency, based on values given by
Kolben. The densities are low, and lie between 40,000 and
20,000 lines per square inch throughout the range of fre-
quencies commonly met with in practice. This curve gives
the density in the core proper; the density in the teeth
of the primary and secondary may be double these valuer

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32 DESIGN OF ALTERNATING §22

without making the hysteresis loss very large, the volume
of the teeth being small. Motors are also commonly built

in which the mag-
netic density will
be found less than
that given by the
J curve, but the val-

^ ues shown should

I, not, as a rule, be

I exceeded. Induc-

5 tion motors, like

I alternators, are

•3 generally built

I' with several poles,

I so that the mag-

» netic flux is sub-

I divided. The

I required cross-sec-

I tion of iron in the

yoke is therefore
small, and a low

MoanstU densiiie. pr Jn^eHon Mda^ magnetic density

may be used with-
o u t making the
machine very heavy. The eddy-current loss in the primary,
like that in transfoi;fner cores, can be kept down to a very
small amount if thin disks are used. The thickness of
stampings used is about the same as for alternator arma-
ture cores, namely, from .012 inch to .018 inch.

SECONDARY CORE LOSSES, MAGNETIC DENSITIES, ETC.

35, The core losses in the secondary are usually quite
small. This is due to the fact that the frequency of the
reversals of magnetism in the secondary is low. If the
armature were standing still, the slip between primary and
secondary would be 100 per cent., and the frequency of the
magnetic cycles in the secondary would be the same a3 io

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§22 CURRENT APPARATUS 33

the primary. When, however, the motor is running under
normal conditions, the slip may not be more than from
2 to 5 per cent. The frequency of the magnetic cycles in
the armature will therefore be only from 2 to 6 per cent, of
the frequency in the field, and the core losses will be cor-
respondingly small.

INDUCTION-MOTOR WINDINGS

PRIMARY WINDING

36. The winding on the primary must be so designed that
it will generate a counter E. M. F. equal and opposite to
that of the mains, neglecting, the small drop due to the
resistance of the coils. It is therefore determined in a man-
ner similar to that used for the calculation of the primary
winding for a transformer. In sohie of the earlier forms of
induction motors the coils were wound on salient poles, but
in modern machines they are placed in slots in the same
way as windings for alternator armatures. Most induction
motors are of the two-phase or three-phase type, and the field
winding of such machines is carried out in the same way as
the winding for the armature of a two-phase or three-phase
alternator. The primary winding may be concentrated or
distributed, the latter arrangement being most generally
used for machines operating at moderate pressures. We
may write for induction-motor windings

£= Joi — X>& (9)

as explained in connection with alternator windings. In
this formula

E = E. M. F. generated by or impressed on each phase;

T = number of turns connected in series per phase;

^ = maximum total magnetic flux from one pole;

« = frequency (cycles per second) ;

ir = a constant depending on the style of winding used.

45— n

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34 DESIGN OF ALTERNATING §22

For a concentrated winding, that is, one with one group
of conductors per pole per phase, k = 1. For a uni-
formly distributed two-phase winding, k = .90. For a
uniformly distributed three-phase winding, k = .95. If the
winding is only partially distributed, the value of k will lie
between the values just given and 1. It will be noticed
that for a given value of the flux, frequency, and number
of volts applied, the number of turns required for a dis-
tributed winding is but slightly more than that required for
a concentrated winding, the difference being about 10 per
cent, for a two-phase motor and 5 per cent, for a three-
phase. The distributed windings are preferred, because
with them there is less magnetic leakage between the
primary and secondary; this decreases the inductance and
improves the power factor of the motor. Generally, the
primary slots occupy about one-half the circumference of
the primary core, as this arrangement allows a fair amount
of space for the windings without forcing the density in the
teeth too high.

37. The cross-section of the conductor used for the pri-
mary winding is determined by the full-load current that the
motor takes in each phase. The relation of this current to
the full -load current taken from the mains will, of course,
depend on the way in which the different phases are con-
nected up. The primary is usually stationary, and cannot
therefore radiate its heat as readily as if it were revolving.
For this reason, the current density should be kept as low as
possible without making the space occupied by the windings
too large. Induction-motor fields usually present quite a
large radiating surface, and are, moreover, generally sup-
plied with air ducts, through which a draft is caused by the
armature. If it were not for this, the allowance per ampere
would have to be considerably more. The circular mils
allowed per ampere varies greatly in different makes of
machines. In some it may be as low as 600 or 600, and in
others it may be 1,100 or 1,200. Much depends on the way
in which the machine is ventilated, but it is always best to

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§22 CURRENT APPARATUS 36

make the allowance as large as possible without interfering
with the design in other respects.

38. The primary winding may be made up of bars or
coils, depending on the voltage at which the machine is to
operate, coils being used on most machines of moderate
size. These are arranged in the same way as has already
been described for two-phase and three-phase armatures, and
what has been said as regards the insulation, etc. of such
armatures applies also to induction-motor primaries. The
primary winding is very often arranged in two layers, form-
wound coils being used.

SECONBART WINDING

39. The number of conductors used for the secondary
winding is largely a matter of choice. The motor may be
built with any ratio of transformation, that is, with any
ratio of primary to secondary conductors, and work well.
It is desirable, however, to make the resistance of the
secondary low, and to get as large a cross-section of copper
as possible into the slots. For this reason, it is usual to pro-
vide the secondary with only one or two bars to each slot,
the space taken up by insulation being thus reduced to a
minimum. The bars are generally rectangular in section,
though in some machines
round bars have been
used.

40. The secondary
conductors are in many
cases grouped into a reg-
ular two-phase or three-
phase bar winding. It is
necessary to use a wound
secondary of this kind
when it is desired to insert resistance in series with the sec-
ondary, either for the purpose of securing a good starting
torque or regulating the speed. When this is done, the

PlO. 17

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36 DESIGN OF ALTERNATING §22

winding is connected up according to the Y method, and
the three terminals brought to collector rings, as shown in
Fig. 17. The three phases /„ /„ and/, are thus connected
to the three resistances r„ r„ and r„ as shown. When the
motor is being started, the phases are connected to the
points a^ b^ and r, and the resistance is gradually cut out as
the motor runs up to speed.

41. When it is not desired to insert resistance in the
secondary circuit, a plain squirrel-cage winding may be
used. There is in this case only one bar in each slot, all of
them being connected by copper short-circuiting rings at
each end of the armature. The squirrel-cage construction
gives a durable and efficient armature, because the winding is
extremely simple, and the end connections between the bars
are of very low resistance. Since the voltage generated in
an induction-motor secondary is very low, the insulation
between the bars and core need not be heavy, as the danger
of burn-outs is almost nil and short circuits do not count
for much, because the bars are short-circuited by the end
connecting rings. Usually, the number of slots in the
secondary is different from the number in the primary,
though this is not absolutely necessary. The use of a dif-
ferent number of slots tends to avoid any dead points at
starting, and prevents the motor from acting merely as a
static transformer with a short-circuited secondary.

POWER FACTOR

42, It is important that the pcwep factor of an induc-
tion motor be high, otherwise it will take an excessive
amount of current for a given amount of power delivered, on
account of the angle of lag between the current and E. M. F.
In order that the power factor may be high when the motor
is loaded, the magnetic leakage and consequent inductance
must be kept low. This may be done by using a small air
gap, subdivided windings, and slots that are partially opened
at the top.

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22 CURRENT APPARATUS 37

liBNGTH OF AIR GAP

43. The current necessary to set up the magnetic flux
through the field will be largely dependent on the leng^tli
of alp gap between the primary and secondary, because
this constitutes by far the greater part of the reluctance of
the magnetic circuit. In a transformer it is not necessary
to have any air gap in the magnetic circuit; hence, the
magnetizing current can be made quite small. In an induc-
tion motor, however, an air gap is unavoidable, and all that
can be done is to reduce this to the smallest possible amount.
The air gap is therefore made as small as the necessary
mechanical clearance will permit. For very small motors
the single air gap may not be more than yj^ inch. For
larger machines it must be greater than this, on account of
the difficulty of centering large armatures exactly, and to
prevent the armature touching the field in case the bearings
should wear slightly.

GENERAL. DATA

44. The following figures, given by M. A. C. Eborall,*
will serve as a guide for the values of some of the various
items entering into the design of induction motors. These
apply for the most part to motors designed for the ordinary
frequencies of 50 to 60 cycles. These must be taken as a
general guide only, and individual machines might show
values differing considerably in some particulars from these
and yet give good results.

46. Perlplieral Speeds. — From 4,000 to 7,000 feet per
minute. The speed of large motors is usually somewhat
higher than that of the smaller machines.

46. Number of Poles. — Two to 7^ horsepower, 4 poles;
10 to 30 horsepower, 6 poles; 40 to 100 horsepower, 8 poles.

47. Full-lK>ad Efficiency. — Table II gives ordinary
values for the full-load efficiency.

* London Electrician, 1900.

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DESIGN OF ALTERNATING

TABIiE

Brake Horsepower

Polyphase motors

Single-phase motors

•75
.72

•79

•75

•85
.80

.87
•83

.90
•85

. 48. Full-Lioad Power Factor. — Table III gives ordi-
nary values for the full-load power factor.

TABIiB

Brake Horsepower

Polyphase motors - -

.78
.72

.80

•75

•85
.80

■87
•83

.88

Single-phase motors. . .

•85

49. Tjength of Air Gap. — The following values
(Table IV) give the minimum length of air gap that it is
safe to use for mechanical reasons. In some machines
larger air gaps than these are employed. The lengths
given refer to a single gap only.

TABLE IV

Rotor Diameter

Air-Gap Length
Inch

Between 5 inches and 8 inches

Between 9 inches and 12 inches ,

Between 15 inches and 20 inches

Between 24 inches and 32 inches

Between ao inches and 60 inches

60. I>enslty of MaKnetisni In Stator Teeth. — Table V
gives values for the density in the stator teeth.

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CURRENT APPARATUS

■69

TABIiE V

Horsepower

Density in Lines
Per Square Inch

2 to 7 . C

65,000

. 70,000

80,000

lo to to

40 to 100

Above 100

85,000

The density in the air gap should not exceed 30,000 lines
per square inch, and is usually considerably lower than this.

61. Density of Magrnetlsm in Rotor Teeth. — Table VI
gives values for the density in the rotor teeth.

TABIiE VI

Horsepower

Density in Lines
Per Square Inch

2 to 7 . s

80,000

10 to 30

85,000

90,000

100,000

40 to 1 00

Above 100

62, Cnrrent Densities per Square Inch. — With low
and medium pressure semi-enclosed motors, the amperes per
square inch cross-section of stator or field conductor will be
between 1,500 and 1,100, according to size. This corre-
sponds approximately to 850 to 1,150 circular mils per
ampere. With high-tension motors, somewhat smaller
values must be taken on account of the space occupied by
the insulation.

63. Volume of Current in Stator and Rotor. — The

ampere-conductors, i.e., the product of the current and

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DESIGN OP ALTERNATING

conductors, for each inch periphery should have values
about as shown in Table VII.

TABLE VII

Horsepower

Am pere-Conduc-
tors Per Inch
of Periphery

2 to 7 . «:

250
330
430
570
600

lo to 30

40 to 100

100 to 150

Above 200

64. Slip at Full Lioad. — Table VIII gives approximate
values of the slip at full load in per cent, of synchronous
speed.

TABIiB Vm

Horsepower

Slip
Per Cent.

2 to «c

7
5

74- to IK

20 to 40

50 to 100

DESIGN OF 10-HOR8EPOWER MOTOR

66, In order to illustrate the design of a simple induc-
tion motor, we will take an example and make the calcula-
tions required for the windings and core. Many of the
mechanical details are similar to those that have already
been described for alternators, so that they need not be
taken up in detail; those parts that differ materially will be
described as the design is worked out. We will take for an
example a 10-horsepower three-phase motor with stationary
primary and revolving secondary. The primary will be

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§22 CURRENT APPARATUS 41

provided with a distributed winding placed in slots, the
secondary being provided with a squirrel-cage winding.
We will suppose that the following quantities are given:
Output at pulley, 10 horsepower; line voltage, 220 volts;
frequency, 60 cycles per second ; power factor at full load, .85 ;
commercial efficiency at full load, 85 per cent.

FULL-IiOAD CTTRRENT IN PRIMARY

66. The output is to be 10 horsepower, or 10 X 746
= 7,460 watts = IV, The actual power to be delivered to

the motor at full load will therefore be ' ^ = 8,776 watts

.85

= IV'.

The true watts delivered to the motor at full load are
equal to the product of the volts and amperes into the power
factor cos 0, where ^ is the angle of lag between the cur-
rent and E. M. F. We then have

true watts .^^.

apparent watts = — (lO)

cos 0 = .85 in this case; hence, we have

apparent watts = ^^^ = 10,324 = IV"
.85

For a three-phase motor we have
W" = E/V^
;rhere £ is the voltage between the lines, and / the current
in each line ; hence,

10,324 = 220X /X |/3
j^ 10.324 y,

220 X >/3

The full-load current in the line will therefore be

27.1 amperes, and the current in each phase will also

be 27.1 amperes if we adopt a Y winding for the primary.

If we used a A winding, the current in each phase would

27 1
be —j= = 15.7 amperes, nearly.

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42 DESIGN OF ALTERNATING

SIZE OF PRIMABT CONBUCTOB

67. Since the current in each phase is comparatively
small, we will use the Y method of connection for the pri-
mary winding. The current in the primary conductor will
therefore be 27.1 amperes. We will provide 850 circular
mils per ampere as a fair allowance of copper for the
primary. We then have 27.1 X 850 = 23,035 circular mils.

A No. 6 B. & S. has a cross-section of 26,251 circular
mils, and three No. 11 wires in multiple give a cross-
section of 24,702 circular mils. Two No. 9 wires in parallel
will give 26,188 circular mils, so that any of these arrange-
ments would give the requisite cross-section. When it
comes to arranging the dimensions of the slot, a decision
can be made as to which arrangement can be used to best
advantage.

PERIPHEBAL SPEED AND BIAMETEB OF ABMATUHB

68. If the speed of rotation and the frequency are fixed,
the number of poles for which the field must be wound is at
once determined; or, if the number of poles and frequency
be fixed, the speed of rotation at no load at once follows,
because at no load the speed of the armature is almost
exactly equal to that of the revolving field, the slip being
very small. If we wind the field so as to have six poles, the

speed at no load will be very nearly s = r — -^—p — n —

^ -' -^ number of poles

2 X 60
= — - — = 20 revolutions per second, or 1,200 revolutions

per minute. If the field were wound for eight poles, the
speed would be 900 revolutions per minute. As this motor
is not very large, 1,200 revolutions per minute will be a fair
speed for it. If we used the eight-pole arrangement, we
would obtain a lower speed, but the motor would be larger
and more expensive; we will therefore adopt the six-pole
1,200-revolution arrangement.

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§22 CURRENT APPARATUS 43

59. Induction motors are run at moderately high periph-
eral speeds, usually between 4,000 and 7,000 feet per min-
ute, the larger motors having the higher peripheral speed.
For a motor of the size under consideration, 4,500 feet per
minute will be a fair value. The outside diameter of the
armature will therefore be

^ peripheral speed x 12 ^ 4 500 x 12 ^ ^^ 3^^ .^^^^^
" R. P. M. X ^ 1,200 X w

We will therefore adopt 14| inches as the outside diameter
of the armature. The circumference of the armature will
be about 45.16 inches. The inside diameter of the field
will be equal to the outside diameter of the armature plus
the air gap required for mechanical clearance. For an
armature of this diameter ^ inch on each side should be
sufficient, so that the inside diameter of the field will be
14| + 2 X Vf = I^tV inches. The inside circumference of
the field will be about 45.35 inches.

PRIMART WINDING

60. We will use a primary wlndingr that is subdivided.
If the winding is subdivided to a large extent, a large num-
ber of slots will be required to accommodate it. It is
usually sufficient, however, for motors ranging from 10 to
100 horsepower, to use from two to four coils per pole per
phase, and for the present case we will take three coils per
pole per phase as a trial arrangement. The winding will be
arranged in two layers ; hence, there will be as many slots as
coils. The number of slots will therefore be 3 X 6 X 3 = 54.

61. Before fixing upon the size of the slots, it will be
necessary to determine the number of conductors. We will
design the primary so as to make the ampere-conductors per
inch of periphery as nearly 300 as possible, as this should
give good results for a motor of this size. The circumfer-
ence of the stator is 14^ X 3.1416 = 45.35 inches; hence.

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DESIGN OF ALTERNATING

the total ampere-conductors will be 45.35 X 300 = 13,605.
The current in each conductor is 27.1 amperes; hence,

13,605

number of conductors should be

27.1

= 502, approxi-

mately. There are 54 slots, and as the winding is to be
arranged in two layers, there must be an even number of
conductors per slot, so that the nearest number will be 10.
This will make the nearest total number of conductors 540.

63, In order to obtain a slot that will be fairly deep
compared with its width, we will use three No. 11 wires in

^ air gap

eirewMnff
ring

multiple, with a cotton wrapping on each wire. The diam-
eter of the wire over the insulation will be .101 inch, and

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§22 CURRENT APPARATUS 45

allowing 65 mils on each side for slot insulation, taping, and

clearance, the width of the slot will be 3 X .101 + 2 X .065

= .433 inch. The space taken up by the 54 slots will

be 54 X .433 = 23.38 inches, thus leaving 45.35 — 23.38

= 21.97 inches for the teeth. Each tooth will therefore

21 97
be ' = .407 inch wide at the circumference. This is
54

not much less than the width of the slot, and will give
ample cross-section of iron to carry the flux, because the
density in the tooth will not be much more than twice that
in the air gap, and as the latter will not be more than
20,000 to 25,000 lines per square inch, there will be no dan-
ger of the teeth becoming saturated.

The slot must have sufficient depth to accommodate
10 wires in addition to the slot insulation, the dividing
insulation between the upper and lower layers of coils, and
the wedge or fiber strip used to hold the coils in place. We
will allow ^ inch for the middle insulation, and -^^ inch
for the holding in strip. The total depth of the slot will
then be 10 X .101 + 2 X .065 + ^^ + ^ = 1.390, or, say,
l^-} inches, in order to allow a small amount for clearance.
The dimensions of the slot and the arrangement of the
ten turns of three-wire conductor are shown in Fig. 18, the
coils being held in place by wooden or fiber strips slipped
into notches in the teeth.

MAGNETIC FLUX IN POL.ES

63. By the ma^rnetic flux ^ is meant the total max-
imum number of lines that flow from one pole piece. The
pole pieces of an induction motor are not sharply defined
like those of an alternator field, but gradually merge from
one into the other.

Fig. 19 will help to convey an idea as to the way in which
the flux is distributed around the face of an induction-motor
field. The inner circle represents the face of the field,
which for the present will be considered as unbroken by
slots. If a current is sent through the windings, six poles

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DESIGN OF ALTERNATING

Pig. 1»

will be formed, as shown, and these poles will be continually
shifting around the ring. We will consider the instant

when the centers of
the poles are at the
points marked iV, S.
The magnetic den-
sity is greatest op-
posite the center of
the pole, and may be
represented by the
arrow a b directed
outwards from a
south pole, or a' b'
directed inwards
from a north pole.
As we move away
from a pole the
field intensity d i -
m i n i s h e s until it
becomes zero at the point midway between the poles, and
begins to increase again in the opposite direction. This vari-
ation in the magnetic density at the various points of the pole
face is represented approximately by a sine curve, and if
the line a b represents the maximum value of the density,

2
the average value of the density will be ab X -, since the

average value = maximum value X -. Hence, if B repre-

sents the maximum value of the density, B X -■ will be the

average density. The total flux ^ is equal to the area of the
pole face multiplied by the average value of the density; or

* = arc ef X length of field parallel to shaft X B X -

. /. ^ X diameter of field

Arc ef = 1 > -~.

number of poles

- -. TT X diameter of field , ., r /< ,j » 2

hence, ^ = - - ^^ v r , - - X length of field X B X -;
' number of poles ^ -n

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§22 CURRENT APPARATUS 47

or we may write, for the length of field parallel to the shaft,

'-2xar,xB ^"^

where ^ = flux from one pole;
/ = number of poles;
df = inside diameter of field ;
B = magnetic density in the air gap (maximum).

Hence, from th^ formula we can obtain the length of the
field parallel to the shaft when we know the value of ^ and
have decided on the air-gap density to be used. The other
quantities in the equation are already known. We can
obtain the value of the flux from the formula

^ = — lo^x^

We will take ^ = .95, as the winding is nearly imiformly

distributed. There are eighteen coils in each phase, with

5 turns each, so that the number of turns T in series per

phase is 90. The voltage generated in each phase will be,

220
neglecting the resistance drop, —-= = 127 volts, because the

armature is Y connected. We then have

4.44 X * X 90 X 60 X .95

127 =

10"

127 X 10'
^-^ * = 4. 44 X 90 X 60 X. 95 = ^^"^'^^ "''''"' approximately

64. The magnetic density in the air gap should not be
forced too high, or a large magnetizing current will be
required to set up the flux. From 20,000 to 30,000 lines per
square inch may be taken as fair values for the air-gap den-
sity. The density at the top of the teeth would of course
be more than this. We will take 20,000 lines per square
inch in this case. Applying formula 11, we have for the
length of the core parallel to the shaft, the field diameter
being 14tV = 2^3^ inch.

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48 DESIGN OF ALTERNATING §22

^ 557,500 X 6 X 16 _
^ - 2 X 231 X 20,000 - ^'^^ '""^^^^

The length of the iron part parallel to the shaft should
therefore be, say, 5|J inches, in order that the air-gap den-
sity shall not exceed 20,000 lines per square inch. The
length of core over all will be somewhat greater than this,
owing to the space taken up by insulation between the disks
and by the air ducts if the latter are used. We will allow
I inch for an air duct in the center of the core, and ^ inch
for the space taken up by the insulation, thus making the
spread of the laminations over all 6|^ inches.

65, All the dimensions of the primary have now been
determined except the depth of the iron under the slots,
that is, the dimension d^, Fig. 18. This must be made
such that there shall be a sufficient cross-section of iron to
keep the magnetic density down to the proper amount.
Referring to the curve, Fig. 16, we find that a fair value
for the magnetic density in the iron of a 60-cycle motor is
about 30,000 lines per square inch. The magnetic leakage
in such a motor is small, and we may take the flux in the
field as practically the same as that in the air gap. The
flux through a cross-section of the yoke under the slots will
be I 4>, because the flux from one pole will divide, one half
flowing in one direction and the other half in the other
direction. The area of cross-section of iron in the yoke
will therefore be

which gives

A, = '-S§^ = 9.29 square inches

The actual length of iron parallel to the shaft is 5fJ inches;
hence, the depth of iron under the slots must be

9 29
^c = g otlg = 1.6 inches, nearly

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§a^

CURRENT APPARATUS

[-J^^

4^?^^

1 1

r 1

_ . . . 1

^ fog

45—12

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50 DESIGN OF ALTERNATING §22

We will therefore make the dimension d^^ Fig. 18, 1| inches.
The inside diameter is 14^ inches, and the depth of the slots
\\\ inches, so that the outside diameter of the stampings
for the primary will be 14^ + 2 X Iff + ^ X if = 20J inches.

The complete dimensions of the primary are shown by
(^), Fig. 20. A section through one of the primary slots is
given at (b)^ showing the air duct b and a section of the
laminations. The primary laminations are provided with a
keyway k for holding the stampings in place and bringing
the slots into line. There will be 64 slots of the dimensions
shown in Fig. 18, equally spaced around the inner periphery.

SECONDART WINDi:5^G

66, The design of the secondary follows largely from
that of the primary. The outside diameter is already
known, and the length of the secondary core over all par-
allel to the shaft will be the same as the length of the
primary, 6^-J inches. We will provide the secondary with a
squirrel-cage winding, although a secondary with a regular
three-phase Y winding might be used if it were desired to
insert resistance when starting. It is advisable, though
not absolutely necessary, to use a number of slots for the
secondary different from that used in the primary, as it
tends to prevent dead points at starting. We will there-
fore try 60 slots for the secondary winding, and see if this
number gives a satisfactory design in regard to the size of
the slots and bars.

ROTOR CONDUCTORS AND CORE

67, The magnetizing action of the currents in the
secondary of an induction motor is, at each instant, equal
and opposite to the magnetizing action of the currents in
the primary, as is the case in an ordinary transformer.
The total volume of current in the secondary may then, for
purposes of calculation, be taken equal to that in the pri-
mary. In this case we have a total of 540 stator conductors

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§22 CURRENT APPARATUS 51

carrying a current of 27.1 amperes. Hence, the total
volume of current is 540x27.1=14,634 ampere-con-
ductors. If, therefore, we use 60 bars on the armature,
the current in each bar will be approximately ^^ij^
= 243.9 amperes. The voltage that must be generated in
the secondary at full load in order to set up this current in
the bars will depend on the resistance of the bars, the
higher the resistance, the greater being the necessary
E. M. F. and the greater the slip. It is desirable, therefore,
in order to secure close speed regulation and high efficiency,
to make the resistance of the bars as low as practicable.
The core losses in the secondary are very small on account
of the low frequency of the magnetism in the secondary, so
that as far as heating is concerned, we might allow a large
I^ R loss in the conductors; an allowance as low as 300
or 400 circular mils per ampere would not likely give rise to
any undue heating. We will, however, allow 500 circular
mils per ampere, as this larger cross-section will tend
toward better speed regulation and higher efficiency. The
cross-section of the secondary bars will then be 243.9 X 500
= 121,950 circular mils = .096 square inch, nearly. The
usual practice is to make the secondary slots for squirrel-
cage armatures rather broad and shallow, as shown in
Fig. 18. This brings the conductors near the surface of
the rotating member, and also allows the bars to be placed
in the best position for connecting to the end short-circuit-
ing rings. The distance between centers of the secondary

slots will be ' = .753 inch, or a little over | inch. A
dO

bar y\ inch by ^ inch has a cross-section of very nearly
.096 square inch; a bar of these dimensions will be placed in
the slot as shown in Fig. 18. A bar of this size will have a
cross-section of approximately 121,800 circular mils, allow-
ing a little for rounding the corners. The width of the bar
is ^ inch = .438 inch; hence, there is .753 — .438= .315 in'ch
left for the tooth and the insulation. This will allow the teeth
to be made ^y inch projected width at the circumference
and still leave sufficient space for insulation. Since the

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52 DESIGN OF ALTERNATING §22

voltage generated in the secondary is very low, a light slot
insulation is all that is necessary. In this case there will be
room enough for .017 inch insulation around the bar. The
secondary slots are made nearly closed at the top, as shown
in Fig. 18, and the bars are pushed in from the end.

68, The bars are connected up into closed circuits by
means of the short-circuiting rings r, Fig. 18, one at each
end of the armature, the bars being bolted to the copper
rings by means of the flat-headed countersunk bolts s. In
order to secure good contact, the projecting ends of the
bars .should be milled to conform with the surface of the
ring. The lower the resistance of the end rings, the better,
but as the path of the current through these rings is short,
there is little advantage gained by putting a large amount
of copper into them. We will make the thickness of the
rings the same as that of the bars, i. e., ^\ inch, and will
make the rings | inch wide, in order to secure a good con-
tact between them and the bars.

69, The complete dimensions of the stator and rotor
have now been determined with the exception of the inner
diameter of the rotor disks. The flux through the rotor
will be practically the same as that in the stator. The rotor
might be worked at a higher magnetic density than the
stator without serious loss, because of the low secondary
frequency. However, we will use the same density in both,
so that the depth of iron under the secondary or rotor slots
will be 1| inches. The total depth of the slots is | inch, so
that the inner diameter of the rotor is 14:| — 2 (| + 1|)
= 10| inches.

HEAT L.OSSES

70, The principal dimensions have now been deter-
mined, and it remains to be seen whether the motor will
deliver its rated output without overheating. In order to
do this, we will make an approximate estimate of the

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§22 CURRENT APPARATUS 53

PR losses. The PR loss in the secondary may be deter-
mined approximately as follows: The cross-section of each
armature bar as finally adopted will be about 121,800 circu-
lar mils. The bars should project a short distance out of
the slots, so we will call the length of each bar about
8^ inches. The hot resistance of each bar will then be

„ lenofth in inches 8.5 ^r.r.r.^r. t

R = — ?--, -. — = ri;,-o,^, = .000069 ohm

circular mils 121,800

The total PR loss in the armature will be (243.9)'
X .000069 X 60 = 246 watts. There will also be a certain
amount of loss in the short-circuiting rings and at the
joints, but the total PR loss will probably not exceed
300 watts. The outside cylindrical surface of the armature
is 45.16 X 6.687 = 302 square inches, nearly, which gives
a surface of over 1 square inch per watt /' R loss. The
core losses in the secondary will be very small, so that the
secondary, will carry its load without any danger of over-
heating.

71, In order to estimate the /' R loss in the primary at
full load, we must first determine the length of a primary
turn. There are in all 54 coils and 54 slots, the coils being
arranged in two layers. There are six poles, so that if one
side of a coil lies in the top of slot No. 7, the other side will
lie in the bottom of slot No. 10, as shown in the winding
diagram. Fig. 22. The coil will then span over ^^ of the
circumference of the field, as shown in Fig. 21. This figure
represents two coils of the field winding in place, the inner
face of the field being developed out flat. When the coils
are in place, the ends a, a and b, b will project out past the
core, forming a cylindrical winding. The ends of the coils
are arranged on such a slant that they will fit in as shown
without crowding. From this layout of the coils, the length
of an average turn can be obtained, and in the present case
it is found to be about 36 inches. There are 18 coils
in series per phase and 5 turns per coil, making a total of
90 turns. The cross-section of the conductor is 3 X 8,234

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DESIGN OF ALTERNATING

= 24,702 circular mils, since there are three No. 11 wires in
parallel. The resistance per phase will therefore be

R =

90 X 36
24,702

= .131 ohm, nearly

The /*R loss per phase will then be (27.1)' X .131
= 96.2 watts, and the total /* R loss in the field will be
96.2 X 3 = 288.6, say, 290 watts. The exposed cylindrical

Pio. Zl

surface of the field core alone is 20^X3.1416x611
= 430.7 square inches. The surface exposed by the pro-
jecting windings will be approximately 200 square inches,
so that there is an effective radiating surface of 630.7 square
inches for getting rid of the heat developed in the primary,
without counting the radiating surface that would be pro-
vided, to a certain extent, by the frame of the machine in

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FlQ

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pio^a

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§22 CiURREt^t APPARATtfS 65

contact with the field. The radiating surface as a whole,
therefore, should be sufficient to get rid of the losses with-'
out an undue rise in temperature, especially as the hyster-
esis loss in the primary core would not be as large as the
/• R losses, the density being low and the volume of iron
comparatively small.

FTEIiD WINDING AND CONNECTIONS

72. Fig. 22 shows the arrangement of the primary or
field winding, one phase being drawn in complete. The
groups of conductors for the other two phases are indicated
by the light and dotted lines, the connections between them
being made in the same way as those for the phase drawn in.
The rules governing the connecting up of such a winding
have already been explained in connection with polyphase-
alternator armatures. Each of the heavy outlined figures
represents a field coil of 5 turns; the lighter lines (two to
each coil) projecting from the inner point of the coils rep-
resent the terminals of the coils. There are 54 slots, or
9 slots corresponding to each pole; hence, the E. M. F.'s in
all the conductors in the 9 slots under any one pole will be
in the same direction, as shown by the arrowheads. For
example, the E. M. F.*s in the conductors in slots 7, 8, 9, 10,
11, 12, 13, H, 15 will all be in one direction, .say directed
from the front to the back, while those in slots 16, 17, 18,
19, 20, 21, 22, 23, and 2^ will have their E. M. F.'s in the
opposite direction, corresponding to a pole of opposite polar-
ity. The 18 coils shown belonging to one phase must all be
connected in series, so that the E. M. F.'s in the conductors
in the different slots belonging to this phase will be summed
up. Suppose we start with the terminal 7^,; we will pass
five times around the coil, bridging from slot 4.6 to slot /, in
agreement with the arrowheads, and come out at /; we will
connect / to /', and go five times around the next coil, finally
coming to x and completing the connections of that group
of coils. We then pass on to the next group, con-
necting X to y (so as to agree with the arrows), and so on

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56 DESIGN OP ALTERNATING §22

around the field until the whole 18 coils are connected in
series, finally coming to /,. We will connect T^ to the
common connection of the Y winding, T^ being then one of
the terminals of the motor that is connected to the line.
The other two phases are connected up in exactly the same
way, the connections between the terminals of the different
phases and the common junction being made according to
the rules already given. This winding could also be con-
nected up A, the only difference being in the connections
of the phase terminals with each other and with the
terminals of the machine.

MECHANICAL. CONSTRUCTION

ARMATURE

73. The armature core is built up in almost exactly
the same way as cores for alternator or continuous-current
armatures, the disks being mounted on a spider and clamped
together by means of end flanges drawn up and held in place
by capscrews or bolts. If a wound secondary is used, it is
customary to provide the- spider with projecting flanges for
supporting the winding, as already explained for alternator
armatures with distributed windings. Where the squirrel-
cage construction is used, no supports are necessary, the
bars and short-circuiting ring being stiff enough to hold
themselves in place.

SHAFTS

74. Shafts for induction motors are usually made excep-
tionally heavy, considering the power that they must trans-
mit. They should, in general, be heavier than the shafts
used for alternators of corresponding speed and output.
The air gap in induction motors is so small that a very stiff
shaft is required, the slightest bending of the shaft being
sufficient to either let the armature touch the field or bring
very heavy magnetic pulls on the shaft, due to the shorten-
ing of the air gap on one side. The shafts for these motors are

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PlO.23

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§22 CURRENT APPARATUS 57

shorter than those required for alternators and continuous-
current machines, because no room need generally be
allowed for collector rings. Fig. 23 shows the induction
motor that has been worked out. This will give an idea
as to the style of shaft used for such machines.

FIEL.I> FRvVME, BEDPLATE, ETC.

75, The arrangement of the parts of an induction motor
of this size will be understood by referring to Fig. 23. In
this case the field frame forms the main supporting casting
of the machine, being provided with feet as shown. It
serves the double purpose of supporting the field stampings
and forming a base for the machine. In some of the larger
sizes of induction motors, the field frame is bolted to a
separate bed in the same manner as shown for the field of
the alternator. For machines of moderate size, the con-
struction shown in Fig. 23 answers quite well, and is
cheaper than that which makes use of a separate bed. The
self-oiling bearings are carried by the two end plates //, //,
which are bolted to the field frame, as shown, and carry the
bearings g^ g and the shaft /, with {)ulley /. These end-
bearing supports also serve to protect the field coils c. The
conductors in the field slot are shown at d^ d, and ^ is a
section of the field laminations. The armature laminations a
are supported by the spider e and held by the cap bolts and
end flange, as shown. The armature bar is shown pro-
jecting from the slot, the ends being bolted to the short-
circuiting rings. The field frame k is provided with a
number of ribs r, which are bored out to fit the outer cir-
cumference of the stampings. A number of openings o
are cored in the frame to allow ventilation. The terminals
of the field winding are led through the cored openings/, />
to the terminals ;/, which are mounted on the slate terminal
board ;;/, from which the connections to the line are made.
It will be seen that, on the whole, the construction of such a
motor is very simple, there being no brushes, brush holders.

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DESIGN OP ALtfiRNATlNG

2-i

collector rings, etc. Fig. "Zl shows a perspective view of an
induction motor of the same general type as the one worked

Pig. 24

out. The main mechanical features of Fig. 24 will be
understood by referring to Fig. 23, so that further comment
is unnecessary.

76. Two-phase and single-phase induction motors are
designed in the same way as three-phase machines, the only
essential difference being in the arrangement of the wind-
ings. The calculation of two-phase armature windings has
already been described, and the calculations for a two-phase
induction-motor field are made in the same way.

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ELECTRIC TRANSMISSION

INTRODUCTORY

1. Electric transmission may be defined as the trans-
ferring of power from one point to another by means of
electricity. The power so transmitted may be used for any
of the numerous applications to which electricity is now
adapted, such as operating motors, lights, electrolytic plants,
etc. The distance over which the power is transmitted may
vary from a few feet, as in factories, to many miles, as in
some of the modem long-distance transmission plants.

2. A power-transmission system consists of three essen-
tial parts: {a) The station containing the necessary dyna-
mos and prime movers for generating the electricity; {d) the
line for carrying the current to the distant point; and (c) the
various receiving devices by means of which the power is
utilized.

3. Electric transmission may be carried out by using
direct current, alternating current, or a combination of the
two. Generally speaking, in cases where the transmission
is short, direct current is used, though alternating current
is now also largely used for short-distance transmission,
as, for example, in driving factories. When the distance is
long, it is necessary to use alternating current. In cases
where the distance is long and where alternating current
is not well adapted to the operation of the receiving devices,
the current transmitted over the line is alternating, but it is
changed to direct current at the distant end and there dis-
tributed, thus forming a combination of the two systems.
The special applications of electric transmission to railway

For noiiee of copyright, see Ptue immediately tollowinz tfu tHU pQ£$
128

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2 ELECTRIC TRANSMISSION §23

and lighting work will be taken up later in connection with
those branches of the subject; for the present, the object
is only to bring out important points relating to the sub-
ject of electric-power transmission generally.

Power transmission is extensively used in connection with
water powers that would in many cases be of little use on
account of their being located away from railways or com-
mercial centers. It is also coming into extensive use in
factories to replace long lines of shafting and numerous
belts, which are wasteful of power. Its most important use,
however, is in connection with the operation of electric rail-
ways, where the power is transmitted from the central sta-
tion to the cars scattered over the line. The style of
apparatus used will depend altogether on the special kind of
work that the plant is to do, and the type best adapted for a
given service will be described when the different transmis-
sion systems are treated later. Power stations will be taken
up by themselves; the present Section will be confined to
the methods and appliances used for carrying out electrical
transmission.

POWER TRANSMISSION BY DIRECT
CURRENT

4. Up to within a comparatively recent date, electric
transmission for power purposes was carried out by means
of the direct current, alternating current being used when
the power was required for lighting purposes only. Later,
however, alternating-current motors and rotary converters
came into use, and at the present time, large transmission
systems use alternating current for both light and power.

5. Dynamos and Motors Used. — Direct-current dyna-
mos may be of either the constant-current or the constant-
potential type. Practically all the current is distributed at
constant potential and in America compound-wound dynamos
are generally used. The motors used in connection with
such constant-potential systems are generally of the shtmt
or compound type.

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§23 ELECTRIC TRANSMISSION 3

6. Simple Po-wer - Transmission System. — About
the simplest possible
example of electric-
power transmission is
that shown in Fig. 1.
Here a compound-
wound dynamo A is
driven by means of
an engine not shown,
and sends current

through the motor B I

by means of the lines |
My M, The dynamo
is driven at constant
speed and its series-
winding is adjusted
so that the pressure at
the terminals of the
dynamo rises slightly
as the current in-
creases, due to the , . ^
increase of the load
on the motor. This
slight rise in voltage
is to make up for the
loss in pressure in the
line, as will be ex-
plained later. The
pressure at the motor
remains nearly con-
stant, no matter what ^^'^ \Mwncf
load the motor may ^*"^
be carrying, but the
current supplied in-
creases as the load

is increased. When ^'®'^

both lights and motors are operated, such a system will
probably use a pressure of 110 or 220 volts at the receiving

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4 ELECTRIC TRANSMISSION §23

end of the circuit; if used for power alone, a pressure of
250 or 500 volts will be employed. It should be mentioned
that when the receiving: end of a circuit is spoken of, the
end distant from the station is meant, because this is the
end where the various devices, such as lamps, motors, etc.,
receive their current.

7, liost Power and liine Drop. — In order that a
transmission plant may be efficient, the generating apparatus,
line, and motors must be efficient. Dynamos and motors of
good make are generally satisfactory as regards efficiency,
and the question is. How efficient can the line be made ? In
answer to this, it might be said that the loss of power in the
line could be made as small as we please if expense were no
consideration. All conductors, no matter how large, offer
some resistance to the current and there is bound to be some
loss in power. By making the conductor very large we can
make the loss small, because the resistance will be low, but
a point is soon reached where it pays better to allow a cer-
tain amount of power to be lost rather than to further
increase the size of the conductor. The pressure necessary
to force the current over the line is spoken of, in power-
transmission work, as the drop in the line, because this
pressure is represented by a falling oflE in voltage between
the dynamo and the distant end of the line.

8, If R is the resistance of the line and / the current
flowing, the drop is, from Ohm's law, e = I R. The power,
in watts, lost in the line is /Rx/= PR. The power
lost, due to the resistance encountered by the current, reap-
pears in the form of heat. The power generated by the
dynamo is equal to the product of the pressure generated
by the dynamo and the current flowing; or, if E^ repre-
sents the dynamo pressure, then

watts generated = IVi = E^ I ( 1 )

The power delivered at the end of the line is equal to the
product of the pressure at the end of the line multiplied by

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§23 ELECTRIC TRANSMISSION 6

the current, or, if E^ represents the pressure at the distant,
or receiving, end, then

watts delivered = IV, = E^/ (2)

It should be particularly noted at this point that the cur-
rent / is the same in all parts of the circuit. Thus, in
Fig. 1 the same current flows through the motor that flows
through the dynamo, tmless there is a leakage at some point
between the lines, and this would not be the case if the lines
were properly insulated. What does occur is a drop or loss
in pressure between the station and the receiving end, but
there is practically no loss in current except, perhaps, in a
few cases where the line pressure is exceedingly high or the
insulation unusually bad. This point is mentioned here
because the mistaken idea that there is a loss of current in
the line is a common one.

9, We have already seen that the number of watts lost
in the line is given by the equation W =^ P R,

The lost power must also be equal to the difference
between the power supplied and the power delivered, or
jr= JT, - ^., =^. /-^./, =/(^,-^.).

Ex — E, represents the loss of pressure, or the drop, and
it is at once seen that the greater the drop, the greater the
loss in power. For example, a 5-per-cent. drop in voltage
is equivalent to a 5-per-cent. loss of power in the line.

10, In order to transmit power, we must be willing,
then, to put up with a certain amount of loss, or what is
equivalent, with a certain amount of drop in the line. The
amount of drop can be made anything we please, depending
on the amount of copper we are willing to put into the line.
The percentage of drop allowed is seldom lower than 5 per
cent, and not often over 15 per cent, except on very long trans-
mission lines; 10 per cent, is a fair average. In cases where
the distribution is local, as, for example, in house wiring, the
allowable drop from the point where the current enters the
building to the farthest point on the system may be as low

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6 ELECTRIC TRANSMISSION §23

as 1 or 2 per cent. If the drop is excessive, the pressure at
the end of the line is apt to fluctuate greatly with changes of
load and thus render the service bad. In a few special cases
there may be conditions that warrant the use of an excess-
ive drop, but in general the above values are the ones com-
monly met with.

11. When the loss, or drop, in a circuit is given as a
percentage, this percentage may refer either to the voltage
at the station end of the line, or the voltage at the receiving
end. For example, suppose we take the case where the per-
centage loss refers to the voltage at the station end, and let

Ex = voltage at dynamo;

Et = voltage at end of line;

% = percentage loss (expressed as a number, not as a

decimal);
e = actual number of volts drop in the line.

Then, B, - j^#^ (3)

Example. — The voltage at the end of a lighting circuit is to be 110
and the allowable drop is to be 3 per cent, of the dynamo voltage,
(a) Wha: will be the dynamo voltage? (bi) What will be the actual
drop, in volts, in the circuit?

Solution.— (a) We have E^ = _ - = 113.4. Ans.

{b) The drop e = -^?^— ^J- - HO = 3.4 volts. Ans.
lUU — o

12, It is frequently more convenient to express the loss
as a percentage of the power delivered at the end of the line.
For example, if the voltage at the end of the line were 110,
and the loss were to be an amount equivalent to 3 per cent,
of the power delivered, instead of 8 per cent, of the power
generated, it would mean that the allowable drop was 3 per
cent, of 110, or 3.3 volts, instead of 3.4 volts. Railway
generators are commonly spoken of as being adjusted for

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§23 ELECTRIC TRANSMISSION 7

10 per cent, loss when they are wound so as to generate
500 volts at no load and 550 volts at full load; i. e., 50 volts,
or 10 per cent, of 500, is allowed as drop in the line, 500
being the voltage at the end of the line. In expressing the
loss as a percentage, then, it should be distinctly understood
as to whether this percentage refers to the power generated
or the power delivered, otherwise there is liable to be con-
fusion. The best way is to express the drop directly in
volts and then there can be no doubt as to what is meant.
In what follows, we will, when expressing the loss as a per-
centage, refer to the power delivered unless it is otherwise
specified, as this method is now very generally followed.

lilNE CAIiCUIiATIONS

13. Calculations for Two- Wire System. — We are now

in a position to look into the method of determining the
size of wire necessary for a given case. First consider
the simple transmission system, shown in Fig. 1. The
problem of determining the size of a line wire usually comes
up about as follows: Given a certain amount of power to be
transmitted over a given distance with a given amount of
loss; also, given the required terminal voltage; determine
the size of line wire required. The whole problem of deter-
mining the size of line wire simply amounts to estimating
the size of wire to give such a resistance that the drop will
not exceed the specified amount. All the formulas for this
purpose are based on Ohm's law, and are simply this law
arranged in a more convenient form to use. There have
been a large number of these formulas devised, each for its
own special line of work, and the one that is derived below is
given because it is as generally applicable as any.

14. In the first place, if the watts or horsepower to be
delivered and the voltage at the end of the line are given,
we can at once determine the current, because

/ = f (5)

45—13

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8 ELECTRIC TRANSMISSION §23

in which IV, is the power delivered. Furthermore, the
drop e in the line is known or specified, and since

e = /R (6)

or -^ = y, the resistance R of the line is easily determined.

15. Referring to Fig. 1, it is seen that the total length L
of line through which the current flows is twice the distance
from the dynamo to the end of the line. It has already been
shown that the resistance of a wire is directly proportional
to its length and inversely proportional to the area of its

cross-section, orR = , where A' is a constant that depends

on the units used for expressing the length L and the area
of cross-section A. In practice, it is generally most con-
venient to have the length L expressed in feet and the area A
in circular mils. When these units are used, the quantity A'
is the resistance of 1 mil-foot of wire; i. e., the resistance
of 1 foot of wire tsW inch in diameter. If the area ^f cross-
section of the wire were only 1 circular mil, it is evident that

the resistance of L feet of it would be KL, and if the area

/^r

of the wire were A circular mils, its resistance would be .

The resistance of 1 mil-foot of copper wire, such as is used
for line work, may be taken as 10.8 ohms. This resistance
will, of course, vary with the temperature and also with the
quality of the wire used, but the above value is close enough
for ordinary line calculations. The following formula may
then be used for calculating the resistance of any line:

R = ^^4^ (7)

where R = resistance in ohms;

L = length of line in feet (total length, both ways);
A = area of cross-section in circular mils.

16. What is usually desired is the area of the wire
required for the transmission, not the resistance, and by
combining formulas 6 and 7 this can be obtained.

Digitized by VjOOQIC

§23 ELECTRIC TRANSMISSION

We have

e = IR,

but

„ 10.8 L,
^= A '

hence,

lO.S LI

'~ A '

. _ 10.8L/

(8)

Expressing this formula in words, the required area of
cross-section in circular mils

__ 10.8 X length of line in feet X current in amperes
drop in volts

This rule for determining the size of wire for a given
transmission may be written as follows:

Bule. — Take the continued product of 10.8, the total length
of the line in feet, and the current in amperes; divide by the
drop in volts, and the result will be the area of cross-section
in circular mils.

17, It will be noticed that the size of wire has been
determined by making it of such dimensions that the drop
will not exceed the allowable amount. In other words, the
drop has been made the determining factor and no attention
has been paid to the current-carrying capacity of the wire.
If the distance were very short and the drop allowed were
large, the size of the wire as given by the formula might be
such that it would not carry the current without greatly
overheating. This is an important consideration where
wires are run indoors, because the distances are then short
and the rise in temperature of the wire needs to be carefully
considered, owing to the fire risk. This point will be taken
up in connection with interior wiring. For line work such
as we are now considering, the distances are usually so long
that the size of wire as determined by the allowable drop is
nearly always much larger than would be necessary to carry
the current without overheating.

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ELECTRIC TRANSMISSION

§23

18. The formula just given is also often written in the

form

21.6 Z>/

A =

(9)

where D is the distance (one way) from the station to the
center where the power is delivered. Evidently, D is only
one-half the length of wire through which the current flows;
i. e., Z, = 2 D\ hence the constant 21.6 is used instead of 10.8.

19. Formulas 8 and 9 may be applied to a large number
of cases if care is taken to see that the proper values are
substituted. The length L or distance D must always be
expressed in feet. The use of the formulas will be illus-
trated in connection with the following examples. Table I,
giving the area in circular mils of the various sizes of wire
according to the Brown & Sharpe gauge, is here inserted for
convenient reference in connection with the examples.

TABIiE I
SECTIONAL. AREA OF B. 4ft 8. WIRES

No.

Cross-Section

No.

Cross-Section

B.&S.

Circular Mils

B.&S.

Circular Mils

0000

211,600

8.234

000

167,805

6,530

133.079

5,178

105,535

4,107

83,694

3,257

66,373

2,583

52,634

2,048

41,742

1,624

33,102

1,288

26,251

1,022

20,816

810

16,509

642

13,094

509

10,381

404

No.

Cross-Section

B.&S.

Circular Mils^

320

254

202

160

127

lOI

79-7

63.2

50.1

39.7

31.5

25.0

19.8

15.7

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§23 ELECTRIC TRANSMISSION 11

Example 1. — In Fig. 1 the pressure at the receiving end of the
line is to be 500 volts, and 40 kilowatts is to be transmitted with a
drop of 50 volts. The distance from the station to the end of the line
is 3 miles. Calculate the cross-section of wire necessary and give the
nearest size B. & S. that will answer.

Solution.— 40 K. W. = 40,000 watts; hence, current = m%^
= 80 amperes. The distance from the station to the end of the line is
3 mi., but the current has to flow to the end and back again, so that
the length of line L through which the current flows is 6 mi., or
31,680 ft. Applying formula 8,

. 10.8X31,680X80 c^»t ^o/^ ■ , » i a

A = ^ = 547,430 circular mils, nearly. Ans.

This is considerably larger than any of the B. & S. sizes, so that a
stranded cable would be used.

Pio.2

ExAMPLB 2. — It is desired to transmit 20 horsepower over a line
i mile long with a drop of 10 per cent, of the voltage at the receiving
end. The voltage at the end of the line is to be 110. Find the size of
wire required.

Solution. — 20 horsepower = 20 X 746 watts; hence,

20X746 ,«. ^
current = — rv^ — = 135.6 amperes

The drop is to be 10 per cent, of the voltage at the receiving end;
hence, drop e = — :r^ — = 11 volts. The length Z is 1 mi., since the
distance from the station to the end is i mi., and applying formula 8,

10.8 X 5,280 X 135.6 -^ ^cn • i i i a

A = = 702,950 circular mils, nearly. Ans.

This also would call for a large cable.

Example 3.^Fig. 2 shows a simple transmission system as used in
connection with a street railway. The feeder a c runs out from the
station and taps into the trolley wire xy at the point c. The pressure

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12 ELECTRIC TRANSMISSION §23

between the trolley and track at the point r is to be 500 volts, and
the drop in the feeder is to be 10 per cent, of the voltage at the car
when a current of 60 amperes is being supplied. The current returns
through the track, and we will suppose in this case that the resist-
ance of the return circuit is negligible. Required the cross-section
of the feeder ac^

Solution. — In this case the drop takes place altogether in the
wire a ^, because the resistance of the return. circuit through the rails is
taken as zero; hence, the leng^th L used in the formula will be f mi.,
or 3,960 ft., and not twice this distance, as in the previous examples.

500 X 10
The drop in voltage will be ^ = — ^r^ — «» 50, and since the current

is 60 amperes, we have

. 10.8 X 3,960 X 60 -- ooo • 1 n a
A = j^ = 51,322 circular mils. Ana.

By referring to the wire table, it will be found that this is nearly a
No. 3 B. & S.

20. In makingf line calculations, it seldom happens that
the calculated value will agjee exactly with any of the sizes
given in the wire table. It is usual in such cases to take
the next larger size, unless the smaller size should be con-
siderably nearer the calculated value. Generally, the load
operated on a line always tends to increase, because busi-
ness increases, and it is better to have the line a little
large, even if it entails a slightly greater cost when the
line is erected.

21. Formula 8 may also be used for determining the
drop that will occur on a given line with a given current.
In this case the formula is written,

volts drop = d = i^:^ (10)

Example. — Power is transmitted over a No. 3 B. & S. line for a
distance of 4,000 feet. What will be the drop in the line when a cur-
rent of 30 amperes is flowing?

Solution.— The length of wire through which the current flows
is 2 X 4,000 = 8,000 ft. The cross-section of a No. 3 B. & S. wire is
52,634 circular mils ; hence,

,* A 10.8X8,000X30 .o„ .^
volts drop = 52634 ~

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§23 ELECTRIC TRANSMISSION 18

EXAMPLES FOR PRACTICE

1. A dynamo delivers current to a motor situated 850 yards distant.
The current taken by the motor at full load is 30 amperes, and the
pressure at the motor is to be 220 volts. The drop in the line is to be
8 per cent, of the voltage at the receiving end. Required: (a) the drop
in volts ; (d) the size of the wire in circular mils and also the nearest
size B. & S. j^ f (a) 17.6 volts

' \(d) 93,886 cir. mils.; use No. 0 wire

2. A current of 40 amperes is transmitted from a station to a point
1 mile distant through a No. 0 B. & S. wire: (a) What will be the
drop, in volts, in the wire? {b) How many watts will be wasted in the
wire? . ^ Ua) 43.2

^- \W 1,728

USE OF HIGH PRESSURE

22. By referring to the first two examples in Art. 19,

it witf be noticed that the wire called for is very large,

although the amount of power transmitted is not very

great nor the distance long. Suppose a fixed number of

watts Wm to be transmitted with a given voltage E, at the end

of the line; then, the current that must flow through the

IV
line is -=-. We have seen that the loss in the line is

1*11; i. e., if the current be doubled the loss becomes
four times as great. If, then, the E. M. F. be doubled,
we will be able to transmit the same amount of power with
one-half the current, and hence with one-quarter the loss.
Or, putting it the other way, and supposing that the loss is
to be a fixed amount, we can, by doubling the pressure and
thereby halving the current, use a wire of four times the
resistance. For example, suppose we have to transmit
20 kilowatts at a terminal pressure of 500 volts aild that the
loss in the line is to be limited to 2 kilowatts. The current
would be / = HU^ = 40 amperes, and PR = 2,000 watts,
or 40*^ = 2,000; hence, R = HU = 1.25 ohms. Now, sup-
pose that a terminal pressure of 1,000 volts instead of 500 is
used and that the same amount of power is transmitted with
the same number of watts loss as before. The current will
now be / = ^^W = 20 amperes, and /'R = 2,000 watts, as

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ELECTRIC TRANSMISSION

§23

before. We will then have 20* i? = 2,000; i? = ^^ =
5 ohms.

In other words, for the same amount of loss and for the same
amxnint of power delivered^ the allowable resistance of the line
can he made four times as great if the pressure is doubled. Since
the length is supposed to be the same in both cases, this

means that doublingf the pressure makes the amount of cop-
per required just one-fourth as great. If the pressure were
increased threefold, the amount of copper required would be
one-ninth as great, other things being equal. This may be
stated as follows: For the same amount of power delivered and
for the same loss in power, the amount of copper required for
transmission over a given distance varies inversely as the square
of the voltage.

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§23 ELECTRIC TRANSMISSION 15

23. Edison Tliree-Wlre System. — From the preceding
it is seen that an increase in the voltage results in a large
decrease in the amount of copper required. Incandescent
lighting was first carried out at a pressure of 110 volts,
but this pressure rendered the use of large conductors
necessary, and systems were therefore brought out that would
permit the use of a higher pressure. In street-railway work,
a pressure of about 500 volts soon became the standard,
because this appeared to be the limit to which the voltage
could be pushed for this class of work without danger to life.

The Edison three-wire system allows 'current to be
supplied at 110 volts, although the transmission itself is
really carried out at 220 volts, and therefore results in a large
saving in copper over the 110- volt system. The three- wire
system is shown in Fig. 3. Two compound dynamos A and B
are connected in series across the two lines d e and h k.
Each dynamo generates 110 volts, so that the pressure
between the two outside wires is 220 volts, because the two
machines are connected in series. A third wire, called the
neutraly is connected to the point / between the machines, so
that between the lines de and fg there is a pressure of
110 volts, and between fg and hk 2i pressure of 110 volts also.

24. In order to illustrate the action of such a system,
suppose there are six 32-candlepower lamps on one side
and four on the other, each lamp taking, say, 1 ampere.
A current of 4 amperes will flow from the positive side
of B through the line h k and through the lamps to
the neutral wire. At the same time, a current of 6 amperes
will tend to flow out from the positive pole of A over the
line { g through the left-hand set of lamps and back through
e dy as shown by the arrows. In the neutral wire there is a
current of 6 amperes tending to flow in one direction and a
current of 4 amperes tending to flow in the other direc-
tion, the result being that the actual current is the differ-
ence between the two, or 2 amperes, as shown by the full
arrow; or, looking at it in another way, there is 4 amperes
flowing directly across from hk io de and 2 amperes flowing

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ELECTRIC TRANSMISSION

§2?

from A through the neutral wire fg and back through ed
to A, thus making 6 amperes in the line e d. If the cur-
rents taken by the two sides were exactly balanced, no
current would flow in the neutral wire and there would be
practically a 220-volt, two-wire transmission. In any case,
the current in the neutral wire is the difference between
the currents in the two sides, and its direction will depend
on which side is the more heavily loaded.

26. A three-wire system should always be installed so
that the load on the two sides will be as nearly balanced as
possible. The simplest way to estimate the size of the con-
ductors is to first calculate the size of the outside wires,

SOlamps
32 C/?

'/MU^.-

342 Vb/fs.

220Vo/fsf.

Pio.4

l-X

JOLamps
J2C./?

treating it as if it were a 220-volt, two-wire system. When
motors are operated on the three-wire system, they are usually
wound for 220 volts and connected across the outside lines.
The following example will illustrate the method of calcu-
lating the wires for a three-wire transmission:

Example. — Two dynamos deliver power over a distance of 1 mile
to sixty 32-candlepower lamps, thirty lamps on each side of the circuit,
as shown in Fig. 4. A motor that requires a current of 40 amperes is
also connected across the outside wires. Each lamp requires a current
of 1 ampere, and the pressure at the lamps is to be 110 volts. Calculate
the size of wire required for the two outside conductors if the drop in
pressure is not to exceed 10 per cent, of the voltage at the end where
the power is delivered.

Solution.— The first thing to determine is the current. Thirty
lamps are connected on each side and these lamps are connected in

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§23 ELECTRIC TRANSMISSION 17

multiple, each taking 1 ampere. The current in the outside lines
due to the lamps is, therefore, 30 amperes. The motor is connected
directly across the outside lines; hence, the current due to the motor
is 40 amperes, and the total current in the outside lines is 70 amperes.
The pressure across the outside wires must be 220 volts at the end of
the line, because the pressure at the lamps is to be 110. The drop in
the outside wires is, therefore, 220 X .10 =» 22 volts. The length of the
outside wires is 2 mi., or 10,560 ft. Appljring formula 8,

, ., 10.8 X 10,560 X 70 -„^„ ^qa a
circular mils = ~ = 362,880. Ans.

This would require a stranded cable.

26. The neutral wire is often made one-half the cross-
Section of the outside wires, though practice differs in this
respect. It is seldom, however, made less than one-half,
and in a number of cases it is made equal in cross-section.
Of course, if the load could be kept very nearly balanced at
all times, a small neutral wire would be sufficient, but it is
impossible to keep the load balanced, and hence it is usual
to put in a neutral of at least one-half the cross-section of
the outside wires. In the above example, a No. 000 wire
would probably be large enough for the neutral. For dis-
tributing mains, where there is much liability to unbalan-
cing, the neutral is made equal in size to the outside wires.
In some special cases, three-wire systems are arranged so
that they can be changed to a two-wire system by connecting
the two outside wires together to form one side of the circuit,
the neutral wire constituting the other. If this is done, the
neutral would have to carry double the current in the outside
wires and would be made twice as large as the outside wires.

27, Since the outside wires are only i the size required
for the same power delivered by means of the two-wire,
110- volt system with the same percentage of loss, it follows
that, even if the neutral wire be made as large as the out-
side wires, the total amount of copper required is only
i -t- i, or I of that required for the two-wire, 110-volt system.
The amount of copper in the neutral wire is only i that
required for the two-wire system, because it has i the cross-
section and its total length is i that for the two-wire system.

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18 . ELECTRIC TRANSMISSION §23

28, From the preceding it is seen that the three-wire
system of distribution effects a considerable saving in copper,
owing to the use of a higher pressure. Three-wire systems
operating 220-volt lamps with 440 volts across the outside
wires have been introduced with considerable success, thus
making a still further reduction in copper. The tendency
has naturally been to use as high pressure as possible, but
there are grave difficulties in the way of transmitting cur-
rent at high pressure by means of direct current. These
difficulties may be classed under the heads (a) difficulty of
generating direct current at high E. M. F.; and {d) difficulty
of utilizing direct current at high pressure after it has been
generated.

29, Machines for the generation of direct current must
be provided with a commutator, and this part of a well-
designed machine gives comparatively little trouble if the
pressure generated does not exceed 700 or 800 volts; beyond
this point, it becomes a difficult matter to make a machine
that will operate without sparking. Moreover, in direct-
current dynamos, the armature winding has to be divided
into a large number of sections or coils, and the numerous
crossings of these coils make it exceedingly difficult to
insulate such armatures for high pressures.

30, Even if it were possible to generate high-pressure
direct current, it would be difficult to utilize it at the other
end on account of the danger to life. About 500 to 600 volts
is as high as it has been found safe to operatq street railways,
the consideration of safety setting this limit on the pressure
used. Moreover, it is just as difficult to build motors for
high-pressure direct current as it is dynamos, and for most
purposes the high-pressure current would have to be reduced
to low pressure before it could be utilized with safety at the
distant end of the line. This transformation could be effected
by using a high-voltage motor to drive a low-voltage dynamo.
In some cases, these two machines might be combined into
one having an armature provided with two windings and two
commutators, this armature being arranged so as to revolve

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§23

ELECTRIC TRANSMISSION

in a common field magfnet. The high-tension current from
the line is led into one winding through one commutator
and drives the machine as a motor. The second set of
windings connected to the other commutator cuts across the
field and sets up the secondary E. M. F., thus applying cur-
rent to the low-pressure lines. A machine of this kind is
known as a dynamotor. * It is thus seen that the trans-
formation of direct current from high pressure to low
pressure involves the use of what is essentially a high-pres-
sure, direct-current motor — a piece of machinery that is liable
to give more or less trouble for the reasons already stated.

SPECIAL. THREE-WIRE SYSTEMS

31. The ordinary three- wire system requires two dyna-
mos, and a number of special systems have been devised
whereby a three-wire system may be operated from a single
machine. Some of these systems will be found described in

2*0 v^

uov*

Pio. 5

connection with Electric Lighting, Perhaps the most common
method, outside of the regular system using two machines,
is the use of a single large dynamo connected across the
outside wires and a balancing set consisting of a pair of small

Digitized by VjOOQIC

20 ELECTRIC TRANSMISSION §23'

machines connected in series across the lines to take care of
the unbalanced portion of the load, the neutral wire being
connected between [the machines, as described in Electric
Lighting.

32. Dobrowolsky Tliree-Wlre System. — Fig. 5 shows
a method invented by Dobrowolsky for running a three-wire
system from a single dynamo. ^4 ^ is an ordinary direct-
current armature connected to its conmiutator in the usual
manner. Two diametrically opposite points of the winding
are connected to the rings r, r', and from these connection is
made to the terminals of a choke coil. The coils Cy d have
an equal number of turns, and as they are wound on the
laminated iron core ^, they have a high inductance. The
pressure applied to the terminals of Cy d is alternating, because
connection is made to the armature winding through slip
rings r, r^. Since the E. M. F. applied to Cy d is alternating,
the coils will not short-circuit the armature because of the
counter induced E. M. F. Also, since c and d have an equal
number of turns, the point flfwill always be at a potential
midway between that of the two terminals attached to the
collector rings, and if the neutral wire / is attached to the
junction of c and dy the pressure between / and either outside
wire will be one-half that between the outside wires. If the
system becomes unbalanced, a direct current flows through
/, but the choke coil offers no opposition other than the
slight ohmic resistance of c and dy because this current is
steady and cannot therefore set up a counter E. M. F. Also,
if a direct current flows into the coils through /, it divides,
half flowing through c and half through dy and since the two
parts of the direct current circulate around the core in oppo-
site directions, the magnetizing effect of the direct current is
zero, and it does not therefore interfere with the choking
effect that the coils exert on the alternating current.

33. Fig. 6 shows how this system has been applied by
the Westinghouse Company. In order to get a more uniform
action, the winding is tapped at four points, as in Fig. 6 (^i),
and these points connected to four collector rings in exactly

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§23

ELECTRIC TRANSMISSION

the same way as for a quarter-phase rotary converter, the
commutator and brushes being Jiere omitted. The four
rings Ax, Bi, An, B,, Fig. 6 (d), are connected to the choke
coils C, C and the mid-points x of each coil, or rather pair
of coils, are connected to the neutral wire a. If the choke
coils could be moimted in the armature and revolved with

Pio. 6

it, the connections would be equivalent to those shown in
Fig. 6 {c)y and but one collector ring would be required to
connect the neutral wire with the neutral point O. In some
cases three pairs of choke coils are used connected to six
equally spaced points in a manner similar to that shown in
Fig. 6 (a), each point connecting to a collector ring. The

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ELECTRIC TRANSMISSION

§23

'—P/ufse /-

LOftMWlfiJ

*^f¥iaie2-

LpmqaqjJ

diagrams are here shown for two-pole machines; for multi-
polar machines there would be a connection to each ring for
each pair of poles.

34. Fig. 7 shows a method of operating a three-wire,
direct-current system from two-phase, alternating-current
mains. An arrangement of this kind is useful where the
greater part of the output of a plant is utilized as alternating
current, but where it is desired to use part of it for operating
direct-current motors on the three-wire system or supply an
existing three-wire, direct-current system from an alternating-
current station. A and
B are two transformers
with their primaries con-
nected to the two phases
and their secondaries
connected in series and
feeding a two-phase,
three-wire rotary con-
verter. The mid-point
Cof the two secondaries
is connected to the
neutral wire N. It is
evident that point C is
always at a potential
half way between that

• of the outside wires,
or in other words the
pressure between C and
D or C and E is always
half that between E and
Dy and the pressure between N and F or N and G is half that
between F and G, which is the condition required for a three-
wire system.

35. Direct-Current Converter. — Referring again to
Fig. 5, it will be seen that instead of driving the armature
A by means of a belt and thereby operating a three-wire
system from a single dynamo, the armature may be driven

Pig. 7

Digitized by VjOOQIC

§23 ELECTRIC TRANSMISSION 23

by means of current supplied from an outside source.
When operated in this way the machine acts as a direct-
current converter, and by means of it direct current can
be transformed to another direct current at half the voltage,
or the current supplied can be delivered as another at twice
the original voltage. For example, in Fig. 5, current at
220 volts can be supplied at the brushes and a current of
twice the amount delivered at 110 volts. Or, if current is
supplied at 110 volts to one pair of the three terminal
wires, it will be converted to a current of one-half* the
volume at 220 volts. Direct-current converters have been
used in some cases where it is desired to operate 250-volt
motors from a 500-volt power circuit. These machines have
so far been used but little for this class of work, motor
dynamos or dynamotors having been used instead.

POWEli TRANSMISSION BY ALTER-
NATING CURRENT

36. The difficulties encountered in the generation and
utilization of high-tension direct current led engineers to
adopt alternating current for places where the power had
to be transmitted over considerable distances. At first, alter-
nating current was used for lighting work only, because the
single-phase alternators first introduced were not capable of
readily operating motors, although they were quite satisfac-
tory for the operation of incandescent lamps. With the
introduction of polyphase alternators along with the induc-
tion motor, the use of alternating current for power purposes
became very common, and plants using line pressures as
high as 60,000 volts are in regular operation.

37. Alternating current is well adapted for high-pressure
work, because not only can it easily be generated, but what
is even of greater importance, it can be readily transformed
from one pressure to another. The winding of an alter-
nator armature is very simple, no commutator is necessary,
and the problem of generating high pressures becomes a

46—14

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24 ELECTRIC TRANSMISSION §23

comparatively easy one. In some cases, the current is gener-
ated at a low pressure and raised by step-up transformers
for transmission over the line. At the distant end it is
easily lowered, by means of step-down transformers, to any
pressure required for the work to which it is to be put.

SINGIiE-PHASB TRANSMISSION

38. The simplest scheme for alternating-current transT
mission is that which uses a single-phase dynamo; i. e., a
machine that generates a single alternating current. In
Fig. 8, A represents a simple alternator generating current
at a high pressure. This current is transmitted over the
line to the distant end, where it is sent through the pri-
mary of transformer By which lowers the pressure to an
amount suitable for distribution to the lamps /. The syn-
chronous motor M is operated directly from the line, because
it can be wound for a high voltage. If, however, this high
pressure about the motor should for any reason be objection-
able, step-down transformers could be used. As already
mentioned, such systems are installed for lighting work
almost exclusively. At first a pressure of 1,100 volts at the
alternator, or about 1,000 at the end of the line, was
commonly used. Later, pressures of 2,200 and 2,000 volts
became the ordinary practice. In cases where the distance
was very long, step-up transformers were used, as shown in
Fig. 9. Here the current from the alternator A is first sent
into the primary of the transformer Z", which raises the voltage
to any required amount, with, of course, a corresponding
reduction in current. At the other end, the transformer 7^
steps down the high line pressure to whatever pressure is
suitable for local distribution.

39. The single-phase system has been used in the past to
a limited extent for the operation of synchronous motors.
The ordinary single-phase synchronous motor will not start
up even if it is not loaded and this is a great drawback
to its use. The single-phase system is therefore seldom
installed where power is to be transmitted for the operation

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I
I

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26 ELECTRIC TRANSMISSION §23

of alternating-current motors of large size. The motor M
shown in Fig: 8 is the same in construction as an alter-
nator, but it would have to be provided with some arrange-
ment for bringing it up to speed. It is possible that in the
future single-phase, alternating-current motors may be so
improved that this system will be used much more largely
for power purposes than it is now. Experiments have
already been made in the operation of electric railways by
means of single-phase motors constructed similar to series
direct-current motors, but having laminated fields. The
results obtained have been so satisfactory that a large
increase in the use of single-phase current for power purposes
may be expected, though at present the single-phase series
motor has not been used to any great extent in regular com-
mercial work.

TWO-PHASE POWER TRANSMISSION

40. The great advantage of the two-phase system over
the single-phase is that it allows the operation of rotary-
field induction motors and two-phase synchronous motors.
Fig. 10 shows a two-phase system. In this case, we have
taken the simplest arrangement, where the alternator feeds
directly into the line without the use of step-up transformers.
If, however, the distance is very long, step-up and step-down
transformers could be connected in each phase, in a manner
similar to that shown in Fig. 9. A is the alternator supply-
ing the two currents differing in phase by 90*^ to the four line
wires. B, B are two transformers supplying lights. One is
connected on phase No. 1 and the other on phase No. 2, so
as not to unbalance the load on the alternator. C, C are two
large transformers supplying alternating current at 389 volts
to the rotary transformer D, which changes it to direct current
at 550 volts suitable for operating the street-railway system E,
F, F are two transformers supplying a two-phase induction
motor G, H shows a two-phase synchronous motor. This
is the same in construction as the generator A, and it is not
necessary to use transformers with it, as it can be con-
structed for the same voltage as the generator.

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28 ELECTRIC TRANSMISSION §23

THREE-PHASE POWER TRANSMISSION

41. In the three-phase system, if the load on all three
phases is kept nearly balanced, as it usually is in practice,
only three wires are needed. For the same amount of power,
line loss, and distance of transmission, the three-phase sys-
tem requires only three-fourths the amount of copper called
for by the single-phase or two-phase systems. For this
reason, it is often used for the transmission itself, even if the
power is generated by means of two-phase alternators. By
a special arrangement of transformers, described later, two
currents differing in phase by 90° can be transformed into
three differing in phase by 120°. Fig. 11 is similar to Fig. 10,
except that it is arranged for a three-phase transmission.
There is little choice between the two-phase and three-
phase systems so far as actual operation is concerned, the
chief point in favor of the three-phase system being the
saving in line wire.

42. In many large transmission systems, it is customary
to generate the power in one large central station and
distribute it at high pressure to a number of substations
located at the various distributing centers. At these sub-
stations the current is transformed down and passed through
rotary converters, if direct current is necessary, and dis-
tributed to the various devices to be operated. This is
commonly done in connection with both lighting and street-
railway work. ' If alternating current alone is used, the volt-
age is merely stepped down by means of large transformers.

At present, the three-phase system is the one most largely
used for power transmission purposes. When the power is
used for railway operation, the alternating current is changed
into direct current, because heretofore alternating-current
motors have not proved as satisfactory as direct-current
motors for railway operation, hoisting, or other variable
speed work. However, recent developments in the line of
the single-phase series motor with laminated field seem to
indicate that motors of this or similar type can be built so as

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30 ELECTRIC TRANSMISSION §23

to have sufficiently large output and at the same time run
without sparking. These motors have properties much the
same as series-wound, direct-current motors. They give a
good starting torque and are well adapted to variable speed.
A great deal o{ experimenting is at present being done with
them, and it is probable that the single-phase system will,
in the future, be a strong competitor of the two-phase and
three-phase systems for railway work.

lilNB CAIiClTLATIONS FOR AliTBRNATING
CURRENT

43. The factors that determine the size of line wire for
a direct-current transmission apply also, in a general way,
to alternating-current systems. The resistance of the line
causes a drop in pressure between the station and the dis-
tant end, and the line must be proportioned so that this drop
will not be excessive. If the load to be carried is practically
non-inductive, and if the distances are not long, the same rules
that have already been given for direct-current circuits may
be applied with sufficient accuracy to alternating-current lines.
If, however, the lines are long, say more than 2 or 3 miles,
there are other effects that must be taken into account. It
must be remembered that the current is continually changing,
and this introduces effects not met with in continuous-current
circuits where the current flows steadily in one direction.
The size of wire required will depend not only on the amount
of the load, but also on the kind of load, i. e., on whether it
consists wholly of motors or lights, or a combination of the
two. In direct-current circuits, it makes no difference, so far
as the drop in the line is concerned, how far the wires are
strung apart on the poles, but in an alternating-current circuit
this may have an appreciable effect.

The effects of self-induction and capacity on alternating-
current transmission lines have already been given in con-
nection with the subject of alternating currents. On all but
very long transmission lines the effects of capacity are not
serious, but the inductance of the line may have quite a large

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§23 ELECTRIC TRANSMISSION 31

influence on the line drop. The relation between the line
drop, terminal E. M. F., and generator E. M. F. has been
shown by means of an. E. M. F. diagram, and by laying
out such a diagram, the size of wire for any particular
case could be obtained. For ordinary line calculations,
however, it is convenient to use formulas that may be
easily applied, and that will give results accurate enough
for most practical purposes.

PORMUJLA8 FOR lilNE CAIiCUIiATIONS

44, Estimation of Cross-Section of Lilnes. — In a

direct-current transmission line a certain drop in voltage is
equivalent to a corresponding loss in power. With alter-
nating current, the percentage drop in pressure may be quite
different from the percentage loss in power. In case alter-
nating current is used, the drop in voltage will very likely
be more than the corresponding loss in power, because of
the self-induction of the line. Just what the drop will be,
corresponding to a given loss in power, depends on the size
of the wire, distance apart on the poles, etc. The exact
calculation of line wires for alternating current is a compli-
cated matter, but in nearly all the cases that arise in prac-
tice they can be estimated with sufficient accuracy by means
of comparatively simple formulas. The following fomlulas,
originated by Mr. E. J. Berg, will be found convenient for
estimating alternating-current lines. The different quantities
entering into the calculations are as follows:

D = distance in feet over which power is transmitted
(this distance is to be taken one way only, i. e., it
is the single distance);

Wt = total watts delivered at the end of the line (this
number must express the actual watts delivered,
not the apparent watts);

P = percentage of power lost in line (it should be noted
that this percentage is that of the power delivered,
not the power generated; also, it is the percentage
power lost, not the percentage drop in voltage);

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32 ELECTRIC TRANSMISSION §23

E^ = voltage required at the receiving end of the line, i. e.,
the voltage at the end where the power is deliv-
ered;
/ = a constant having the following values:
2,400 for a single-phase system. operating lights only;
3,000 for a single-phase system operating motors and lights;
3,380 for a single-phase system operating motors only;
1,200 for a three-wire, three-phase and four-wire, two-phase

system, all lights;
1,500 for a three-wire, three-phase and four-wire, two-phase

system, motors and lights;
1,690 for a three-wire, three-phase and four-wire, two-phase
system, all motors.

The cross-section of the wire required for any given case
may then be calculated from the following formula:

circular mils = -5-^V ( 1 1 )

Example. — 300 horsepower is to be transmitted by means of the
three-phase system over a distance of 5 miles with a loss of 10 per cent,
of the power delivered. The pressure at the end of the line is to be
4,000 volts and the power is to be used altogether for operating motors.
Calculate the size of line wire required.

Solution. — In this case the distance D is 5,280X5 = 26,400 ft.
The watts delivered will be 300 X 746 = 223,800. P = 10 and i?, = 4 ,000.
The constant/ for this case will be 1,690; hence, we have from formula

. , ., 26,400X223,800X1,690 ^^ .^

circular mils = 10 X 4,000 X 4-:000 " = ^^.407,

or about a No. 2 B. & S. Ans.

45. Estimation of Current in Liines. — The current in
the line wires of an ordinary direct-current line is easily
obtained by dividing the watts delivered by the voltage at the
end of the line. The current in the case of alternating-
current systems can be calculated by using a similar formula
and multiplying by a constant, to allow for the circumstances
under which the current is used, as follows:

current in line = ^- (12)

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§23 ELECTRIC TRANSMISSION 33

where IV, = watts delivered;

£t = voltage at the receiving end of the line;
T = constant referred to above.

Values of Constant T

Single-phase system, all lights 1 .052 *

Single-phase system, motors and lights 1.176

Single-phase system, all motors 1.250

Two-phase, four- wire system, all lights 526

Two-phase, four-wire system, motors and lights .588

Two-phase, four-wire system, all motors 625

Three-phase system, all lights 607

Three-phase system, motors and lights 679

Three-phase system, all motors 725

Example 1. — ' 100 kilowatts is delivered by means of the two-
phase, four-wire system to a mixed load of motors and lights. The
pressure at the receiving end of the line is 2,000 volts. Calculate the
current in each line wire.

Solution.— 100 K. W. = 100,000 watts. For this case the con-
stant Twill be .588; hence,

^ 100,000 X .588 ^ ^
current = — 2 ono " amperes. Ans.

ExAMPLB 2. — 200 kilowatts is transmitted by means of the three-
phase system, the voltage between lines at the receiving end being
4,000 volts. The load consists wholly of motors; calculate the current
in each line.

Solution.— 200 K. W. = 200,000 watts. For this case the value

of T will be .725; hence,

^ 200,000 X .725 ^^ „. .

current = ^7:7^ = 36.25 amperes. Ans.

4,000

46. Estimation of Drop. — The volts drop in the line

P £,

for a continuous-current system would be "77vfr» when P is

the percentage of delivered power lost and E, is the voltage
at the receiving end of the line. This formula can be made
to give the approximate drop in an alternating-current line
by multiplying it by a constant that takes into accoimt the
conditions under which the line is operated, as follows:

volts drop in line = ^^ (13)

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ELECTRIC TRANSMISSION

§23

The value of M depends on the frequency, the power factor
of the load, and the size of the line wire; its value, under
various conditions, is given in the following table:

TABIiE II

Values of M

30 Cycles

60 Cycles

125 Cycles

fid

>»

A
^

>«

•0

•d

■>

o
d
J5

•S

»4

0
0

oooo

211, 6oo

1.26

1.27

1.24

1.64

1.85

2.44

3.06

3.14

ooo

167,805

1.20

1.17

1. 14

1.49

1.63

1.62

2.15

2.62

2.67

133.079

I. 15

1.08

1.05

1.39

1.46

1.42

1.92

2.25

2.29

105,535

r.io

I.OO

1.30

1.32

1.28

1.73

1.96

1.99

83,694

1.06

1. 00

I.OO

1.23

1. 21

1. 16

1.57

1-74

1.73

66,373

1.03

I.OO

1. 16

I. II

1.06

1.44

1.54

1.53

52,634

1.02

I.OO

I. II

1.04

I.OO

1.35

1.38

41.742

1. 00

I.OO

1.07

I.OO

1.26

1.22

33,102

1. 00

I.OO

1.04

I.OO

I. 19

1. 16

I. II

26,251

1. 00

I.OO

1.02

I.OO

I. 14

1.08

1.03

20,816

1. 00

I.OO

1.09

1. 01

I.OO

16,509

1. 00

I.OO

1.06

I.OO

Example. — 600 kilowatts is to be transmitted a distance of 6 miles
by means of the three-phase 60-cycle system. The loss in the line is to
be limited to 10 per cent, of the power delivered, and the pressure at
the receiving end of the line is to be 6,000 volts. The current is to be
supplied to a mixed load of motors and lights. Calculate: {a) the size
of the line wire; [b) the current in each line; (r) the volts drop in the
line; and {d) the pressure generated by the djmamos at full load.

Solution.— (a) 600 K. W. = 600.000 watts. 6 mi. = 6 X 5,280
= 31,680 ft. Using formula 11, we have, since /for this case is 1,500,

, ., 31,680X600,000X1,500 ^^ ,,^

circular mils = ^ ^^-^ ^^^^ ^ ^^^— = 79,200

A No. 1 B. & S. wire would therefore be used. Ans.

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§23 ELECTRIC TRANSMISSION 35

{d) In order to obtain the current in each line we use formula 12,
and for this case, the value of 7" will be .679; hence,

^ 600,000 X. 679 ^_ ^ .

current = — -^ ,^,^ = 67.9 amperes. Ans.

o,UUU

(c) In order to calculate the volts drop in the line, we use formula
13. For a No. 1 wire and a frequency of 60 cycles on a combined
lamp and motor load, the value of the constant Mis found to be 1.21
by referring to the table; hence,

,^ ^ 10X6,000X1.21 .^ .
volts drop = ^ = 726. Ans.

{(f) Since the cjrop in the line is 726 volts, the pressure at the dynamo
must be 6,000 -f 726 = 6,726 volts when the full-load current is being
delivered. Ans.

Note.— In the above example, the drop In the line wonld have been only 600 volts
if continnons current were used.

EXAMPI.E9 FOR PRACTICE

1. 250 horsepower is to be supplied to 60-cycle induction motors by
means of the two-phase, four-wire system over a line 3 miles long. The
pressure at the distant end of the line is to be 4,000 volts and the loss
in the line is to be limited to 8 per cent, of the power delivered. Cal-
culate: {a) the size of the wire required; (d) the current in each line
wire; {c) the drop in the line. f (a) 39,000 cir. mils, nearly;

A«o J about No. 4 B. & S.

^°^|(/^) 29.14 amperes
I (c) 320 volts

2. A three-phase alternator delivers 400 horsepower to a mixed load
of motors and lights. The pressure at the distant end of the line is
3,000 volts. Calculate the current in each line. Ans. 67.54 amperes

3. 6,000 incandescent lamps are supplied with current from a single-
phase alternator, having a frequency of 125, over a distance of 3 miles.
The loss in the line is to be limited to 10 per cent, of the power deliv-
ered, and the pressure at the end of the line is to be 3,000 volts. Allow
60 watts for each lamp supplied and calculate: (a) the size of the line
wire; {d) the current in the line; (r) the volts drop in the line; (d) the

(a) 126,720 cir. mils, or about

No.OOB. &S.
(d) 105.2 amperes
(c) 576 volts
1(d) 3,576 volts

voltage at the generator.

Ans.

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36 ELECTRIC TRANSMISSION §23

THE SELECTION OF A STSTEM

47, From the foregoing it is seen that the engineer has
a large number of systems to choose from when installing a
given plant, and the selection of a system for any given case
is a matter that requires careful consideration. We will,
therefore, endeavor to sum up the principal advantages and
disadvantages of the different systems as an aid in determin-
ing the system to be used in any given cas6.

The selection of a system, so far as its bearing on the
location of the station is concerned, is comparatively unim-
portant in ordinary street-railway work, as the 500-volt,
direct-current system is the standard American practice, due
allowance being made for distance. But in the case of
lighting and power distribution over large districts, and for
long-distance railway work, the problems require careful
analysis.

DIRECT-CURRENT SYSTEMS

48. If lighting and motive power are required, the first
points to be considered are the characteristics of the town
and nature of the business to be expected. In compactly
built, thickly settled places, where a good site for a station
can be had within a mile from the most distant lights or
motors, there is no better or cheaper system, either in first
cost, economy, or convenience of operation than the direct-
current system, and whether it should be two- or three-wire,
circumstances will determine. Where distances exceed 1 mile,
boosters can be used advantageously, or the double-bus
system of high and low potential. These last two arrange-
ments are described more in detail later. In the follow-
ing we will state the potential on the system of distribution,
and due allowance must be made for drop in E. M. F.
between generators and the point where the energy is utilized.

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§23 ELECTRIC TRANSMISSION 37

49. The two-wire, 220-volt system is in successful
operation, and the 220-volt incandescent lamp is perfected
for use on a commercial basis. There can be no question of
the gjeat advantage of a 220-volt, two-wire system over the
three-wire system in simplicity and reduced cost of copper.
It must be recognized, however, that greater care is required
in insulating and installing all interior fittings that require
more or less handling.

50. Tlipee-Wlre, 220- Volt System. — ^The advantages
of the three-wire, 220-volt, direct-current system are many,
among which may be mentioned the following; some of these
also apply to the 220-volt, two-wire system.

1.' Low potentials in dynamos, station apparatus, and
street lines, and consequent perfect safety to the dynamo
attendants, linemen, and the public.

2. Greatly lessened leakage, and therefore reduced risk
from fire. .

3. Convenience, cheapness, and ease of connection to the
wiring on the consumers* premises.

4. The reading at the station, of pressure returned from
extreme feeder ends by means of pressure wires, as described
later, indicates quite accurately the pressure at the consumers*
premises.

5. As the dynamos are run in parallel on the system in
conjunction with station methods of regulation and control,
it is possible to tie the mains and feeders together wherever
convenient, thus insuring by equalization a more uniform
pressure, no matter to what extent the electrical center or
heavy load in the district may shift during the 24 hours. By
enabling the lightly loaded lines to supplement those that
are heavily loaded, this system of intermeshing conductors
equalizes the potential and gives the best results from a
given weight of copper.

6. The use of direct current makes possible the employ-
ment of storage batteries as an adjunct to the central station,
thus lessening the hours during which it may be necessary to
operate a considerable portion of the steam plant, minimizing

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38 ELECTRIC TRANSMISSION §23

the labor account, and enabling one to run the boilers,
engines, and dynamos at a higher efficiency during the period
they are in operation, and to shut them down as soon as the
load is low enough to justify throwing all or a portion of it
on the storage battery. Moreover, in case of a sudden or
heavy demand for extra current, such as may be occasioned
by bad weather or sudden thunder storm, the battery is
always on hand, ready to be thrown on instantly to supple-
ment the dynamos, whereas it requires some time to start
an idle engine and throw in its dynamos.

7. Electrolytic and electroplating work can be done with
the direct current, but is impossible with alternating currents,
except at considerable expense and complication for rotary
converters or other transforming devices.

8. The measurement of power, calculation of conductors,
and arrangement of circuits are simpler than in the alter-
nating system, on account of the absence of induction and
consequent lag effects.

9. Simple and efficient motors are readily installed and
operated, and form a considerable source of income.

10. The broad establishment of the business, the vast
amount already served by the three- wire system, and its
standardized methods largely influence its adoption.

But the three-wire system has manifest disadvantages, the
most prominent of which are as follows:

1. The two sides of the system must be kept at nearly
equal loads, as want of balance occasions a difference in
potential between the positive and negative sides, and conse-
quently a difference in the brilliancy of the lights.

2. If overhead lines are used for large currents, they are
cumbersome, costly, and extremely liable to disaster from
high winds or lightning.

3. It is impossible to cover a very large extent of territory
at 250 volts potential without great expense for copper.

4. A ground on any part of the wiring, no matter how
trifling in itself, may be a fault on the whole system, and
if not promptly eliminated may give rise to a bad short
circuit.

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§23 ELECTRIC TRANSMISSION 39

5. Switchboard and other connections are complicated
because of the use of three wires and the operation of the
dynamos in pairs connected two in series.

51. Three- wire, 600- Volt System* — ^A larger extent
of territory can be served by the use of * the three-wire,
250-500-volt, direct-current system because it has greater
capabilities of expansion, with less investment in copper for
lightly loaded or scattered territory, as well as requiring less
copper in heavily loaded business districts. The advantage
stated for the 500-volt, three-wire system with the same cur-
rent distribution and the same station location, is that it will
cover, at the .same cost of copper, a territory four times as
large as with a 250-volt, three-wire system. Increased risks
are encountered as mentioned, as regards insulation within
buildings and for underground distributing systems because
of higher potential, but these conditions are successfully met
by the employment of standard appliances. The important
point is that the ignorant consumer shall be fully protected
when current is supplied him at potentials bordering on the
danger line.

AliTEBNATING-CUBBBNT 8T8TEM8

52. The alternating-current system has great value
in the special field of transmission for long-distance and
house-to-house supply in scattered territories, and is excel-
lent and comparatively economical as a temporary expedient
for developing business in a new territory. Before alter-
nating current can be used in compact territory in com-
bination with, or to replace, direct current, the following
improvements are necessary:

1. A type of motor must be developed that will meet all
commercial requirements, which can be used successfully for
all classes of business without causing disturbance of the
fixed potential of the system.

2. A universal system of supply that does not require
transformers or anything except a meter to be located on
the premises of the customer.

45—15

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40 ELECTRIC TRANSMISSION §23

3. Some type of apparatus that will replace the storage
battery as used in connection with direct current.

Alternating current cannot be used in connection with
storage batteries, except through the employment of a rotary
converter or motor generator for charging the battery. The
use of such converting apparatus will be justified when the
amount of current supplied and compensation received is
sufficiently large to overbalance the extra cost for special
equipment and the losses incurred for conversion of
energy.

The direct-current motor can be better applied for general
power work, and in some respects is superior to the alter-
nating-current motor in its electrical operation. The dis-
turbing effects on the system are less, when starting and
stopping large motors. The initial cost of direct-current
motors and their few necessary auxiliaries is much less
than that of alternating-current motors. Alternating-cur-
rent induction motors, on the other hand, have the advan-
tage over direct-current of not requiring a commutator and
brushes. Direct current is best adapted for elevator work.

With direct current at least 80 per cent, of the manu-
factured power can be accounted for through the meters on
a good system, whereas with the alternating-current system,
from 50 per cent, to 60 per cent, only of the power can be
accounted for; the rest is lost in transformers and special
devices.

The comparative usefulness of the two systems for com-
mercial distribution is illustrated in Chicago, where with a
maximum output of 25,000 kilowatts, 20.4 per cent, is for
60-cycle distribution covering a territory of 58 square miles,
and 79.5 per cent, is for direct-current distribution over a
territory of 10 square miles.

The concensus of expert opinion is that the alternating-
current system has not attained the requisite degree of per-
fection for general distribution, in compact territory, though
for long-distance work it is indispensable. In compact terri-
tory it cannot be used with storage batteries; ' the motor
cannot be used for general power purposes. It is therefore

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§23 ELECTRIC TRANSMISSION 41

evident that there is not yet any single ideal system that can
be universally applied to serve all local conditions; special
requirements, the environment of the station, and relative
commercial importance of the various classes of service must
be taken into account in determining what is most desirable
for each given locality.

53. The problem for a combination system may, for
example, be solved as follows:

For incandescent lighting and motive power in the business
and near-by residential districts, the three-wire, direct-current
system, 220 volts.

For incandescent lighting and some classes of motive
power in scattered and long-distance territory, the alter-
nating-current system, 2,300 volts primary; 110 to 220 volts
secondary.

For arc lighting in streets, the enclosed series-arcs on the
alternating-current system.

If the bulk of the power is transmitted over a long dis-
tance, or supplied to a widely scattered area, the two-phase
or three-phase systems would be installed; that is, only one
kind of current would be furnished from the station, and if
direct current were essential for any special purpose, it would
be transformed at the consumers' premises by means of a
rotary converter.

In general, it is well to avoid too great a variety of
apparatus in a station, because it necessitates several sets
of duplicate machines. Considerations of economy are fre-
quently sacrificed in order to make the generating imits in
a given station uniform as to size and output.

FREQUENCrr

54. The choice of a proper frequency in alternating-
current systems is important. The early single-phase plants
were designed for from 125 to 150 cycles, and some poly-
phase machines have been built for these frequencies. The
high inductive effects, troubles in parallel operation, and the

Digitized by VjOOQIC

42 ELECTRIC TRANSMISSION §23

difficulty of obtaining low speeds have caused such high
frequencies to be abandoned in favor of 60 cycles or less.
In polyphase plants, therefore, 60, 40, and 25 cycles have
come to be the standard frequencies. The choice of frequency
should be governed by a careful consideration of the apparatus
to which the plant is to furnish power.

If the alternating current is to be used for lighting pur-
poses only, a high frequency affords the advantage of low
first cost, and such a system might be even single phase.
However, the demand for electric power is now so great
that a low-frequency polyphase system is nearly always used
in modem alternating-current installations. The cost of
transformers, per kilowatt, diminishes as the frequency
increases and this is one of the reasons why high frequency
was used in the early installations when belt-driven, high-
speed alternators were used almost exclusively. With
the introduction of slow-speed, direct-driven machines, low
frequencies became desirable, and the increasing use of
induction motors, synchronous motors, and rotary con-
verters also led to the introduction of lower frequencies.
A frequency of 60 cycles is suitable for incandescent light-
ing, arc lighting, and some motive power. When the
current is used nearly altogether for power purposes, it is
better to use lower frequency; 60 cycles will only be found
satisfactory with synchronous motors, rotary converters,
and similar apparatus when the speed regulation of the
motive power is very good, because of the himting or
periodic surgings in speed that are liable to occur. A
frequency of 40 cycles permits current for both lighting and
power purposes to be supplied to advantage. It is within
the limit of reasonable safety for operating rotary converters
and is the lowest limit for satisfactory working of incan-
descent and arc lights; 40-cycle equipments are not in
general use and should only be adopted after analyzing all
anticipated or existing conditions and finding that 60 cycles
cannot be used with reasonable safety. A frequency of
25 cycles is very commonly used where the current is
supplied wholly for power purposes.

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§23 ELECTRIC TRANSMISSION 43

COST OP CONDUCTORS

55. In order to determine the best potential for a power
transmission, it is necessary to consider carefully the cost of
the transmission circuit. The weight of the electric con-
ductor decreases as the square of the potential employed,
and increases as the square of the distance. Dividing the
potential by the distance gives a convenient figure, which
can be used for all potentials and distances. The curves on
the diagram, Fig. 12, given by the General Electric Company,
furnish a ready means of obtaining the amount of copper
required for a given power transmission. The figures on
the curves indicate volts per mile; i. e., potential of line at
generator divided by distance in miles. The weight of
copper, potential, and line loss are in terms of the power
delivered at the end of the line, and not of generated power.
The curves are correct only for three-phase current with 100
percent, power factor. Two-phase, single-phase, or continu-
ous-current transmission requires one-third more copper.
Five per cent, has been allowed for sag and waste in
weights of copper given.

ExAMPLB. — If copper is worth 15 cents per pound, what will the
cost of copper be for a line (three-phase) to transmit 1,000 kilowatts at
10,000 volts over a line 10 miles long, with a loss of 6 per cent, of the
delivered power?

Solution.— Since 1,000 K. W. at 10,000 volts is to be delivered

,. -^ . , .^. e . , . 10,000 volts

over a hne 10 mi. long with 6 per cent, loss, we have — \^ — -.

^ ^ 10 mi.

= 1,000 volts per mi. Looking on the 1,000- volt curve, we find 5 per

cent, loss corresponds to 57 lb. of copper per kilowatt delivered. 1,000

K. W. X 57 = 57,000. If copper costs 15c. a pound the cost will be

57.000 X 10.15 = $8,650. Ans.

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44 ELECTRIC TRANSMISSION §23

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60
65

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lin€ /ass tn^-ee/tf cffowr dsJtVv^.

PiO. 12

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§23 ELECTRIC TRANSMISSION 45

COMBINED OPERATION OF DIRECT-
CURRENT DYNAMOS

OPERATION OF DTNAM08 IN SERTBB

56. Dynamos are not very often run in series. Perhaps
the most common case is where they are run in pairs of two
in series on the three-wire system. Whenever dynamos are
connected in series, their pressures are added in the same
way as the voltage of two or more cells connected in series,
but the current output is not increased. Series-wound dyna-
mos are sometimes run in series, especially when used for arc
lighting. In this case, the connections are very simple; the
positive pole of one machine is connected with the negative
pole of the other, so that the pressures of the two machines
are added together instead of opposing each other. Gen-
erally speaking, series-wound, shunt-wound, or compound-
wound machines may be run in series with very little
difl&culty; in the case of the last named type, the compound
coils must of course be connected in series in the line. In
most cases, however, the demand is for a large current out-
put rather than for a high voltage; hence, plain series running
is not common, except, perhaps, on arc-light circuits.

OPERATION OF DIRECT-CURRENT DTNAM08 IN

PARAIiliEIi

57. Dynamos, both direct and alternating, are much
more frequently operated in parallel than in series. In
Fig. 13 each machine generates the same voltage, and the
pressure between the lines is the same as if a single
machine were used; i. e., the pressure between the lines is
not increased by adding machines in parallel, but the cur-
rent delivered to the line is increased because the line current
is the sum of the currents delivered by each of the machines.

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46 ELECTRIC TRANSMISSION §23

Each machine is connected through its main switch M^ M'
to the heavy conductors C, D, like terminals of each
machine being connected to the same bar. Each machine,
when so connected, delivers current to the main bus-bars
C, D and thence to the line.

It is not as easy a matter to operate machines in parallel
as in series. It is evident that the voltage of each of the
machines must be kept at the proper amount if the com-
bination is to operate satisfactorily; for, suppose the E. M. F.
of B should fall below that of A, then A would send current
through B and run it as a motor, and B would thus be

Fig 13

taking current from A instead of helping it feed into the
line. There are a number of things that must be taken into
account when machines are run in parallel that do not have
to be considered when they are run separately. Compound-
wound machines are run in parallel more than any other
type in this country, though shunt machines are frequently
run in this way also. Series machines are seldom run in
parallel, for reasons to be given later. We will, however,
first consider the series machine briefly, because the com-
pound-wound machine is a combination of the series- and
shunt- wound machines.

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§23

ELECTRIC TRANSMISSION

SERIES DYNAMOS IN PARALLEL.

58. Suppose two series dynamos are in parallel, as shown
in Figf. 14, and assume that they are delivering current to a
load of some kind and that each machine supplies, say, one-
half of the current. Now, if the E. M. F. of one of the
machines A drops slightly, due to a slight variation in speed
or any other cause, the amount of current delivered by A
will decrease, and thus decrease the field excitation, because
the current through the field coil is the same as the current
delivered by A. This lowering of the field excitation of A
will still further cut down its E. M. F. and matters will go
from bad to worse until, in a very short time, A will be driven
as a motor, unless the belt on the heavily loaded machine •
should slip and thus bring down its voltage. The trouble is

r-^tZi'^^^^JOOOT

' Bu>Bar.

BujBof.

PiO. 14

made still worse by the fact that the extra load thrown on
B will raise its E. M. F., because the field of B will be
strengthened. Moreover, when A is run as a motor, its
direction of rotation will be reversed; and this may result in
considerable damage. It is thus seen that two series
machines connected in parallel, as shown in Fig. 14, will be
very unstable in their action, and it is not practicable to so
operate them.

59. Equalizing Connection. — The unstable condition
just referred to can be remedied by using an equalizing con-
nection, or equalizer, as it is commonly called. This is
shown in Fig. 15, where the wire cd \% the equalizer. It
is a wire of low resistance connecting the points c and d

Digitized by VjOOQIC

ELECTRIC TRANSMISSION

§23

where the series-coils are attached to the brushes; e and /
are the regular terminals of the machine. Now suppose that
the machine B delivers a greater current than A\ part of this
current will flow to the + line through the coil df^ but part of
it will also take the path d-C'-€ through the field coil ce oi

-0/aBar. jg

SMfSof: -^

Pig. 15

machine A, The result is that part of the current delivered
by B helps to keep up the field excitation of Ay thus bringing
up its voltage and equalizing the load between the machines.
If A delivers the greater part of the load, due to a drop in
the voltage of B, then part of the current flows through the
path c-d-f and strengthens the field of B,

SHUNT DYNAMOS IN PARALLEL.

60. Shunt dynamos will operate very well in parallel.
They have two properties that make their parallel operation
a comparatively easy matter. In the first place, they are
capable of exciting their own field no matter whether they
are delivering current to the main circuit or not. In the
second place, their voltage drops slightly with an increase in
the load, and this tends to make their parallel operation
stable. Suppose two shunt machines are arranged as shown
in Fig. 16; A and B are the armatures, 5, S' the shunt field
windings, and r, r' the adjustable field rheostats. Z,, L ' are
switches in the field circuit and J/, M^ main switches con-
necting the machines to the line. Suppose that machine
A is in operation, as indicated by the closed position of
switches L and M, To throw machine B in parallel, it is
run up to speed and the switch L' closed; B will at once

Digitized by VjOOQIC

§23

ELECTRIC TRANSMISSION

begin to pick up its field and run up to. voltage. If the
two machines are generating the same voltage and if their
polarity is the same» as it should be, a voltmeter connected
to blocks i, 2 will give no deflection, because the tendency
of the machine A to send current through the voltmeter
will be opposed by B, This state of affairs can be brought
about by adjusting the rheostat r' until the voltmeter indi-
cates that the voltages of the machines are equal, after
which the switch M' may be closed and the field excitation
of B again adjusted

until the proper share
of the load is carried.
In practice, it is gen-
erally found better
to have the voltage
of B about 1 or 2

per cent, higher than
that of A when the
machine is thrown in.
Very often, when
shunt machines are
arranged for parallel
operation, the field is
connected across the
bus -bars instead of
the armature of each

Pio.16

macliine. When this is the case, the field connection is made
as indicated by the dotted lines ry^ r' y', instead of being
connected as shown by the full lines rx, r' x'. The effect
of this is that the switch M must be closed before A will
pick up, assuming that B is not in operation. If A is running
and B is to be thrown in, then the switch U is closed and
B'^ field is at once excited from the mains, so that B comes
up to voltage almost immediately; after the voltage has
been adjusted, switch M' may be thrown in as before.

61. We will suppose that the two shunt machines, Fig. 16,
are running properly in multiple and will now see whether

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50 ELECTRIC TRANSMISSION §23

their operation will be stable or not. It has already been
seen that the shunt dynamo lowers its voltage as the current
output increases. Now suppose that the voltage of A should
drop slightly on account of a drop in speed or from any
other cause. The tendency will be to throw the bulk of the
load on B^ with the result that B'^ voltage will also drop on
account of the above-mentioned property. The dropping of
B'% voltage will relieve it of part of its load and will make
it divide with A. It is thus seen that there is an automatic
tendency for the load to equalize. Again, suppose that the
load on the line is suddenly increased, and that machine B
takes more than its share of the current; the large current
delivered by B will cause its E. M. F. to drop to more
nearly that of A^ and the load will thus be equalized. If
the voltage of one machine should for any reason become
so low that the other machine runs it as a motor, no harm
is liable to result, because the direction of rotation of the
machine as a motor will be the same as when driven by
the engine as a dynamo. As far as parallel running goes, the
shunt dynamo is satisfactory, but it has been replaced by
the compound machine, because the latter will maintain the
line voltage with an increase of load; whereas, with shunt
machines, the line voltage will fall off, unless the switch-
board attendant cuts out some field resistance.

COMPOUND MACHINES IN PARAXIiEIi

62. Since the compound machine is a combination of
the series and shunt machines, one would naturally infer
that the arrangement for parallel running would be a com-
bination of the two preceding ones. Fig. 17 shows the
connections in their simplest possible form; machines A
and B are of equal size and the equalizer E runs directly
between them; c and / are the + terminals of the machines,
while c d and / e represent the leads, or cables, running to
the switchboard; g h and k I are the negative leads running
to the negative bus-bar h I. There would, in practice, be
a main switch in each of these negative leads, but as they

Digitized by VjOOQIC

§23

ELECTRIC TRANSMISSION

are not essential for the present purpose they have been
omitted. As shown by the full lines in Fig. 17, the shunt
windings of the machines are connected in what is known
as sliort slinnt; i. e.» the shunt field is connected across
the brushes. Sometimes the shunt field is connected in
long shunt across the terminals of the machine or across
the bus-bars. It makes very little difference as to the
performance of the machine which connection is used.

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PlO. 17

Most compound machines are provided with low-resistance
shunts 5, S across their series-coils in order that the degree
of compounding may be adjusted. These shunts should be
adjusted so that the machines, when running separately,
will give the same degree of compounding, which means,
in the present case, that when each machine is delivering
the same current, the voltage generated will be the same,
because we are now assuming that A and B are of equal

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62 ELECTRIC TRANSMISSION §23

size. Another condition that must be fulfilled is that the
resistance between the points a and d must be the same as
between b and e. Since we are, for the present, assuming
that the machines are of the same size and make, the resist-
ance of their series-coils a c and b / will be almost exactly
the same. The resistances of the switchboard leads c d and
/ e must, therefore, be equal; the resistance of the equalizer
E should be as low as possible, and it should never be more
than the leads c d or f e.

Pio. 18b

63. We will now examine the action of the machines
under a varying load. In the first place, if the resistance
between a^ is equal to that between be and the machines
are delivering equal currents, then the drop through a d will
equal the drop through b e and points a and b will be at the
same potential. Since current can only flow between points
at different potentials, there will be no current in E under
such circumstances. Suppose, however, that A delivers a
greater current than B\ then the drop in a ^ will exceed that
in ^ ^ and current will flow through the path a-E-b-i-M'-e
and thus build up the voltage of machine B and equalize the
load. If B delivers more current than A^ the drop in *^

Digitized by VjOOQIC

§23 ELECTRIC TRANSMISSION 63

exceeds that in a d and current flows through the path
h-E-or-c-M-d^ builds up the voltage of A^ and makes A take
its share of the load.

64, In Fig. 17 the equalizer E is shown as connecting
the positive brushes. This is usually the case in practice,
though it would work just as well if both a and b were nega-
tive brushes and c / the negative terminals of the machines.
It is only necessary to see that the equalizer connects those
brushes to which the series-coils are attached, and also to
see that the brushes are of the same polarity on each of the

'< bar
fc/H>oanf

Pio. 19

machines. In some cases, the equalizer wire is run directly
between the machines as shown, but often a third wire is
run from points a and b to the switchboard and there con-
nected to an equalizer bar, as shown in Fig. 18. This
represents a very common arrangement, triple-pole switches
being used; the two outside blades for the + and — leads
and the middle blade for the equalizer. There is a differ-
ence of opinion as to whether it is better to run the equalizer
to the switchboard or run.it directly between the machines, as
in Fig. 17. The most recent practice tends toward running
it directly and placing the equalizer switch near the machine.

Digitized by VjOOQIC

ELECTRIC TRANSMISSION

§23

This undoubtedly makes the connections shorter and thus
leads to better regulation. In such cases, the equalizer
switch is usually mounted on a pedestal near the machine,
as shown in Fig. 19.

65. In some railway plants, especially in those where
large generators are used, the main switch that is on the same
side of the machine as the equalizer is placed on the stand
near the machine alongside the equalizer switch. These two

switches are at practically the same potential, and there is no
objection to placing them near each other. In case this is
done, one of the bus-bars is placed under the floor near the
machines and connected directly to the main switch. This
shortens the connections considerably and makes the equal-
ization of the load closer. It also has the advantage of
simplifying the switchboard connections and avoiding
crowding on the generator switchboard panels. Fig. 20

Digitized by VjOOQIC

§23 ELECTRIC TRANSMISSION 55

shows the arrangement referred to. For lighting: .switch-
boards or for small railway boards, both terminals of each
machine are run to the switchboard. In Fig. 20 the main
connections only have been shown, the shunt coils of the
machines and all minor connections being omitted. The
switches a and b are the equalizer and main + switches,
respectively, the equalizer switch being connected to the
brush to which the series-field c is attached. The + lead
from b connects to the + bus-bar under the floor. Note that
these leads should all be of the same length in order to secure
close equalization. In the case of machines 1 and 2 the leads
are doubled back as shown at ^ in order to make them of the
same length as those running from the more distant machines.
The general method of starting up, say, machine 1 and
throwing it in parallel with others is as follows: See that
all switches on the generator panel of the machine are open,
and get the dynamo up to speed. Then close the equalizer
switch a and the + switch b. Also, close the field switch on
the generator panel. Some of the current furnished by the
other machines will flow through the series-coils ^ , because
the series-coil of machine 1 is in parallel with the other
series-coils. This current in the series-coils will cause the
machine to pick up rapidly, and since the shunt circuit is also
closed, the machine soon comes up to full voltage. The
voltage is then adjusted by means of the rheostat until it is
equal to or a little higher than that of the other machines,
and the negative switch e is then closed, thus placing the
machine in parallel with the others. This method of pro-
cedure applies to the case where the + , — , and equalizer
switches are independent of each other, as is usually the
case in modem installations. When triple-pole switches are
used, as in Fig. 18, all three must of course be closed
together after the machine has been allowed to pick up its
field and has had its voltage adjusted. After the machine
has been thrown in parallel, its load is adjusted by varying
the field excitation. In case the machine is provided with a
circuit-breaker, as is nearly always the case on modem
switchboards, the circuit-breaker should be closed before the

46—16

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56 ELECTRIC TRANSMISSION §23

main switch. If any rush of current then occurs when the
main switch is closed, the circuit-breaker is free to act and
disconnect the machine.

66. Main and Equalizer Cables. — In connecting the
machines to the switchboard, cables of ample capacity should
be used. For most cases it will be sufficient to allow from
1,200 to 1,500 circular mils per ampere. For very large
currents it is advisable to use two or three cables in parallel
rather than a single large cable, as better radiating facilities
are thereby provided. The equalizer should be of the same
size as the main cables. In some cases an allowance as low
as 1,000 circular mils per ampere is made for these main
cables, but the better practice is in favor of a more liberal
cross-section.

67. So far, in all that has been said, the machines were
supposed to be alike in size and general design. Under such
circumstances, there is generally no great difficulty in getting
compound machines to operate properly in parallel. Trouble
is often experienced, however, when it comes to operating
machines of different construction and size. Some field mag-
nets will respond to changes in field excitation much more
quickly than others, and other differehces in design may have
considerable effect on the performance of the machines when
they are run in parallel. With two machines of different size,
the problem is to get the load to divide between them in pro-
portion to their size. For example, suppose a large machine A
is connected in parallel with a smaller machine B, as shown in
Fig 21. Each is supposed to be adjusted so that it gives the
same degree of compounding when operated by itself. Also,
when each machine is delivering its proper share of the
load, the drop between ab must equal the drop between f^.
For example, if / is the full-load current of A, R the resist-
ance between a and b, I' the full-load current of By and R'
the resistance between c and d, then IR must equal F R^.
Now, the resistance of the series-coils cannot very well be
altered in order to bring about the required condition of
affairs, so that the only remedy is to insert resistance of

Digitized by VjOOQIC

§23

ELECTRIC TRANSMISSION

some kind in the leads eb ovfd until the above drops become
equal. This resistance will, of course, be very small and
may be made up of a short piece of heavy German-silver strip
or even an extra amount of cable in one of the leads. In the
figure, it is indicated at x, though it may be necessary to
insert it in the main lead of machine B. The resistance must
be inserted in series with the machine giving the least drop
between the points mentioned above. Many times the
attempt is made to bring about the adjustment by changing

mm%^^^^=^

BusBar +

,rf^=n=?F"

Fio. 21

the shunts 5, s!, but such attempts are useless, because just as
soon as the machines are put in parallel, s and s! are also in
parallel and are practically equivalent to one large shunt
across the fields of both machines. The consequence is that
any change in the shunts affects both machines. The adjust-
ment must, therefore, be made in the main lead between the
series-coil and the bus-bar, and any resistance so inserted
must have the same carrying capacity as the series-coils. A
change in the shunt across the series-coils will change the

Digitized by VjOOQIC

58 ELECTRIC TRANSMISSION §23

compoundins: of the machines as a whole, but it will not better
their condition as regards the correct division of the load.

68* Compocmd Machines in Parallel With. Bhunt
MacUnes. — It is not practicable to run a compound machine
in parallel with a shunt machine. If » for any reason, the com-
pound machine takes a little more than its share of the load,
the strengthening of its series-coils makes it still further over-
load itself, with the result that the field rheostat of the shunt
machine calls for constant attention. The only way to run
this combination satisfactorily is either to cut out the series-
coils of the compound machine, thereby making both plain
shunt machines, or else provide the shunt machine with
compotmd coils.

COMBINED RUNNING OF ALTERNATORS

AliTERNATORS IN SERIES

69. Alternators cannot be run in series unless their arma-
tures are rigidly connected by being mounted on the same
shaft, so that the E. M. F.*s generated by the two machines
will always preserve exactly the same relation with regard
to each other. If the machines are driven separately, the
E. M. F.*s may aid each other at one instant and oppose each
other the next, thus making their operation unstable. There
is, in any event, little occasion for operating alternators in
series; the object of series operation is usually to obtain a
high voltage, and this can readily be generated in a single
alternator, or, if the alternator does not furnish a sufficiently
high voltage, the pressure can easily be raised by means of
transformers.

ALTERNATORS IN PARAIiliEIi

70. Alternators can be operated in parallel, although
they are, as a rule, more troublesome than direct-current
machines. This is especially the case if they are very dif-
ferent in size and design. For example, alternators with the
old-style, smooth-core armatures are hard to nm in parallel

Digitized by VjOOQIC

§23

ELECTRIC TRANSMISSION

with modem machines having toothed armatures. In fact,
in many of the older lighting stations special precautions-
were taken at the switchboard to see that two alternators
should never be thrown in parallel.

71. Alternators are operated in parallel in much the
same way as direct-current machines, so far as connections
are concerned; i. e., they are usually connected to bus-bars
through the intervening main switches. If the alternators
are compound wound, equalizing connections should be used;

Pio.22

but very many are operated with a separately excited
field only and no equalizing connection is necessary, the
whole scheme of connection corresponding more nearly to
the running of shunt-wound machines in parallel.

Suppose two single-phase alternators A and B are con-
nected in parallel. In order that the machines may operate
properly and each take its proper share of the load, it is, of
course, necessary to have their voltages equal or nearly so.
There is another important condition that must also be
fulfilled; the machines must be in synclironlsin. This

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60 ELECTRIC TRANSMISSION §23

means that both machines must run at exactly the same
frequency, for if this were not the case, they would get out
of step. Before two alternators are thrown in parallel,
equality of frequency is the most important condition to
be fulfilled. A slight difference in phase will cause an
exchange of current between the machines, but they will pull
each other into phase if the frequencies are equal.

72. Synchronizing. — The state of synchronism may be
ascertained by means of synclironizing: lamps connected
as shown in Fig. 22. 7", T* are two small transformers
having their primary coils connected to the alternators, as
shown. It should be noted that similar terminals I, 1' are
connected to similar sides of the machines. The secondaries
are connected in series through a pair of lamps /, / and a
plug switch m. If the machines are exactly in phase, termi-
nals S and 5' will have the same polarity at the same instant
and the polarities of 4 and 4! will also be alike. But since
like terminals are connected together, the two secondary
voltages will just neutralize each other, as indicated by the
arrows, and the lamps will not glow. If the machines were
directly opposite in phase, the lamps would light up to full
candlepower. It is evident that by reversing the connections
ot one of the transformers the state of synchronism will be
indicated by the lamps being bright. When machine B is
started and the plug inserted at w, the lamps rapidly fluctuate
in brightness; but as B comes more nearly in synchronism the
fluctuations become much slower. When they have become
as slow as one in 2 or 3 seconds, the main switch M' is thrown
in at the middle of one of the beats when the lamps are dark.
In some cases, the connections are so made that the lamps
are bright when synchronism is attained. Whether the state
of synchronism will be indicated by light or dark lamps
depends simply on whether the transformer secondaries are
connected so as to assist or to oppose each other.

73. Synclironlzliig Tvro-Pliase and Three-Pliase
Machines. — Fig. 22 shows the synchronizing arrangement
for a single-phase machine. For a two-phase or three-phase

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§23

ELECTRIC TRANSMISSION

machine the same arrangement may be used, but care must
be taken to make sure that the transformers T, T* are con-
nected to corresponding phases on each of the machines.
This may be determined by using two pairs of transformers;
i. e., one regular pair, as in Fig. 22, and a temporary pair
on one of the other phases. For example, on a two-phase
machine an arrangement similar to that shown in Fig. 22
should be made for each of the phases, and when the con-
nections are right, each set of phase lamps will light or

nach/rte
A/aS.

Pio. 28

become dark, as the case may be, at the same instant, show-
ing that both phases are ready for parallel operation. After
it is known that the connections are all right, the temporary
pair of transformers may be removed and only one pair used,
as in Fig. 22.

74. Fig. 23 shows a common scheme of connections used
for synchronizing with lamps. In this case the connections
are shown for three machines, each machine being provided

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62 ELECTRIC TRANSMISSION §23

with its plug receptacle fi. One small transformer / is con-
nected across the bus-bars, and the other /' can be connected
to any one of the machines by inserting the plug in its
receptacle. For example, suppose the main switch of machine
No, 1 is closed, as indicated by the dotted lines, and that it is
desired to operate machine No. 2 in parallel with No. 1.
Machine No. 2 would be brought up to speed and the plug
inserted at receptacle 2, thus connecting /' to the machine.
With the connections as shown, synchronism is indicated
when the lamps burn to full brightness, hence the generator
switch of machine No. 2 would be thrown in when the lamps
are at the middle of a beat and at full brightness. The same
arrangement could be used for synchronizing with dark lamps,
the only change being that the synchronizing plug would be
cross-connected, thus making the transformers oppose each
other. Should the alternators generate a low voltage, as
is sometimes the case when they are used in connection
with step-up transformers or for low-voltage work, it is not
necessary to use transformers /, /'. All that is necessary
in such cases is to connect the terminals of the synchronizing
circuit direct to the machines or bus-bars and insert a suf-
ficient number of lamps in series to stand the maximum
voltage applied to them. Another plan in low- voltage work
is to use autotransformers that step down the voltage to an
amount suitable for the lamps.

76. Use of Voltmeter for Synclironlzln^. — As

explained above, lamps have been used very largely in the
past for indicating synchronism, but they are not entirely
satisfactory for this purpose. Lamps do not indicate the
point of synchronism as closely as desirable, especially
when large generating units are involved, and they do not
give any accurate idea as to how much the machine being
synchronized is out of phase or whether it is coming into or
going out of phase. If a large machine is connected to the
bus-bars when out of phase, even by a slight amount, a
heavy cross-current will flow, and this frequently results in
burned switch contacts, to say nothing of possible worse

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i23

fiLECTRIC TRANSMISSION

results. A number of schemes have been adopted for
indicating: the point of synchronism more exactly than is
possible with lamps. Fig. 24 shows an arrangement of con-
nections by which the machine voltmeters are used. If a
voltmeter is connected in the same way as sjnichronizing
lamps, the pressure applied to it at synchronism will be
either zero or double the ordinary pressure, depending on
how the transformers are connected. This would make

34mf Sff<hr0nn0d

<S¥^rl€h

Pia.24

the point of sjnichronism, as indicated by the instrument,
come either at the zero end of the scale where considerable
changes in voltage might make very little change in the
reading, or at the maximum point of the swing where a
considerable change in phase difference is necessary to
cause an appreciable change in the resultant voltage. A
scheme for three-phase systems, devised by Mr. J. E. Wood-
bridge, and shown in Fig. 24, overcomes these objections by

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64 ELECTRIC TRANSMISSION §23

making the voltage applied to the voltmeter at synchronism
the resultant of two E. M. F.'s diflEering in phase by 60°
instead of two that are in phase or 180° out of phase, as
is ordinarily the case. The two transformer secondaries,
connected in series through the voltmeter by means of the
synchronizing plug, are attached to two different phases of
the three-phase system in such a way that their E. M. F.*s
differ in phase by 60°. Thus the resultant E. M. F. applied
to the voltmeter is, when the machines are in phase, equal
to the normal E. M. F., thus bringing the pointer some-
where near the mid-point of the scale. The rate of change
of the resultant E. M. F. due to changes of phase relation
is also high with this connection, thus giving a more accu-
rate indication of the exact instant at which the machines
are in phase.

In Fig. 24 the connections are shown for a pair of high-
pressure alternators, and two potential transformers /, / are
provided for each machine. The junction of the two trans-
former secondaries is grounded, as shown; this not only
simplifies the connections by making the ground serve as
one synchronizing bus, but, what is of more importance, it
precludes the existence of a high pressure between the
switchboard instruments and the ground in case the insula-
tion between primary and secondary should break down.
By using suitable plugs in the receptacles a, by the voltmeter
can be used either to indicate the voltage of the machine, or
for synchronizing purposes; lamps are also provided, as
shown, to indicate synchronism along with the voltmeter.
The plug for the machine that is already in operation
connects points 1 and 4, as shown at /x, and the plug for
the machine being synchronized connects points 2', 4', 5',
as shown at b. This connects voltmeter d in series (by
way of the ground connections) with coils e and h, and the
lamps in series with coils e and^. The E. M. F.'s of e and
h differ in phase by 120°, but the coils are connected in
opposition so that one E. M. F. is reversed with respect to
the other and the two E. M. F.'s which combine to act on the
voltmeter differ in phase by 60°, as previously mentioned*

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§23 ELECTRIC TRANSMISSION 65

The E. M. F.'s of e and g are in phase so that the volt-
meter will indicate normal voltage, and the lamps /' will be
dark at synchronism. When the voltmeter is to be used
in the regular way to indicate the machine voltage, a plug
is inserted that connects the upper contacts i', 2', thus
connecting the voltmeter across the transformer and indicating
the voltage between the outside wires.

76. lilncoln Synclironizer. — Voltmeters and other
devices are used in many ways to indicate synchronism, and
it is impossible to here treat all the different methods. Also,
a number of synchronism ^

indicators, or synchrono-
scopes, have been brought
out; Fig. 25 shows one of
these devised by Mr. Paul M.
Lincoln. The terminals of
the potential transformers
are connected to the binding
posts a a, bb, and when the
incoming machine is in syn-
chronism, the hand h remains
stationary in the vertical
position. If the machine that
is being brought into syn- ^^^ ^

chronism is running too fast,

the hand revolves slowly to the right; if running too slow, it
moves to the left. The following description of the principle
of operation of this instrument is that given by Mr. Lincoln.

Suppose a stationary coil F has suspended within it a
coil A^ free to move about an axis in the planes of both coils
and including a diameter of each. If an alternating current
be passed through both coils, A will take up a position with
its plane parallel to F, If, now, the currents in A and ^be
reversed with respect to each other, coil A will take up a posi-
tion 180° from its former position. Reversal of the relative
directions of currents in A and F is equivalent to changing
their phase relation by 180°, and therefore this change of

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66 ELECTRIC TRANSMISSION §23

180° in phase relation is followed by a corresponding change
of 180° in their mechanical relation. Suppose, now, that
instead of reversing the relative direction of currents in A
and /% the change in phase relation between them be made
gradually and without disturbing the current strength in
either coil. It is evident that when the phase difference
between A and F reaches 90°, the force between A and F
will become zero, and a movable system, of which A may
be made a part, is in condition to take up any position
demanded by any other force. Let a second member of
this movable system consist of coil B^ which may be fastened
rigidly to coil A, with its plane 90° from that of coil A,
and with the axis of A passing through a diameter of B.
Further, suppose a current to circulate through B^ whose
difference in phase relative to that in A is always 90°. It is
evident under these conditions that when the difference in
phase between A and ^is 90°, the movable system will take
up a position such that B is parallel to F, because the force
between A and Fis zero, and the force between ^and /^is a
maximum; similarly, when the difference in phase between
B and F is 90°, A will be parallel to F\ that is, beginning
with a phase difference between A and ^ of 0°, a phase
change of 90° will be followed by a mechanical change in the
movable system of 90°, and each successive change of 90°
in phase will be followed by a corresponding mechanical
change of 90°. For intermediate phase relations, it can be
proved that under certain conditions the position of
equilibrium assumed by the movable element will exactly
represent the phase relations; that is, with proper design,
the mechanical angle between the plane of F and that of A,
and also between the plane of ^and that of B, is always
equal to the phase angle between the current flowing in F
and the currents in A and B^ respectively.

77. Fig. 26 shows the general arrangement of the instru-
ment. As seen from the figure, the construction is similar
to that of a small motor. The field A A is built up of iron
laminations, and is wound with coils /% ^that are connected

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123

ELECTRIC TRANSMISSION

in series and joined to the secondary of the potential trans-
former whose primary is connected to the bus-bars. The
armature core B is of the drum type, and is wound with two
coils C and D that are approximately at right angles to each
other. These coils are connected in series, and their junc-
tion X is connected to the middle ring 2 of three collector
rings mounted on the shaft.
The other two terminals are con-
nected to rings 1 and 5. The
middle ring, through its brush,
connects directly to one terminal
of the potential transformer of
the machine to be synchronized.
Ring S connects to a choke coil
or inductance L\ ring 1 con-
nects to one terminal of a non-
inductive resistance R. The
remaining terminals of R and L
are joined to y and connect to
the other terminal of the poten-
tial transformer. The induct-
ance L and resistance R are
adjusted so that the currents in
the coils C and D differ in phase
by very nearly 90®. The cur-
rent in the coils F, F will lag
nearly 90® behind the E. M. F. E^
because of the high inductance of
the field coils; consequently, the
magnetism set up by the field
will be 90° behind the E. M. F.^.
When the current in coil Z7 is in
phase with the field magnetism, D will swing around until
it assumes the vertical position where its plane is at right
angles to that of the field. The current in D is 90® behind E',
because of the inductance L\ hence, at synchronism the
current in D is in phase with the field magnetism, and
the pointer assumes the vertical position. The current

Fio.26

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68 ELECTRIC TRANSMISSION §23

in C is in phase with E'^ and hence differs in phase from
the field current by 90°; hence, at synchronism no torque is
exerted on coil C if the frequencies of E and E' are equal.
But if E and E' differ in phase by 90°, then the current
in D is at right angles to the field and the current in C is in
phase with the field magnetism; consequently, coil C
assumes the vertical position, and the hand swings around
through 90°. For a phase difference of less than 90° the
pointer assumes an intermediate position. If the machines
do not have equal frequencies, i. e., if the machine being
synchronized is running too fast or too slow, the phase differ-
ence between the field on one hand and C and D on the
other is constantly changing, and, therefore, the pointer will
revolve at a speed depending on the difference in speed of
the alternators. From the direction of rotation, the attend-
ant can tell at once whether the machine being synchronized
requires speeding up or slowing down. The synchronizers
made by the General Electric and Westinghouse companies
operate on the above principle, and are now generally used
instead of lamps or voltmeters.

78, The foregoing will give a general idea as to some
of the methods in common use for indicating synchronism.
As before stated, there are a great many possible arrange-
ments and modifications of the connections, but the prin-
ciples involved are much the same in all of them. Some
devices have been proposed to make the action of syn-
chronizing automatic; that is, to close the main switch
automatically when the point of synchronism is reached
instead of leaving the time of closing to the judgment of the
operator. The object is to prevent the machines from being
thrown together at the wrong time, and although a number
of such automatic devices have been patented, they have
not as yet come into general use. One arrangement for
closing the switch is that patented by Mr. Lincoln in con-
nection with the synchronizer just described. An electrical
contact is arranged so that a circuit will be established
when the pointer is anywhere within an arc, such as a b^

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§23 ELECTRIC TRANSMISSION 69

Fig. 26. This arc represents the amount of phase difference
that is allowable and yet have the machines go together
without making a disturbance. The current through this
electric contact operates a switch or relay that in turn closes
the main switch. It is necessary that the relay shall only
operate when the pointer is revolving at a very low speed;
or, in other words, when contact exists for a considerable
time. This is accomplished by providing the relay with
a dashpot that prevents it from closing unless the current
through its magnet is maintained for an appreciable length
of time. If this were not done, the machines would be
thrown together when their frequencies were tmequal,
because the hand in its revolution would make contact with
the arc and close the circuit. It is only when the hand is
moving very slowly that the switch should be operated.

FBATURSS CONNECTED WITH PARALLEL. OPERATION

79, When two alternators are running in parallel, each
will hold the other in step and they will each run at such
a speed as to give the same frequency; if the alternators
have the same number of poles, their speeds will be exactly
the same. When direct-current generators are operated in
parallel, they do not necessarily run at the same speed and
the load carried by each machine can be varied by changing
the field excitation. When the load is increased, the engine
speed drops a little and the governor admits more steam to
the cylinders, thus increasing the power supplied. In the
case of alternators, the machines are compelled to run at the
same speed, and each alternator will deliver power in pro-
portion to the power supplied to it from its prime mover.
Changing the field excitation will not change the power
delivered; the only effect of changing the field strength will
be to set up local currents between the machines. The field
strength should be adjusted so that, for a given total current
delivered, the current delivered by each machine will be a
minimum; or, so that the sum of the currents as indicated by
the machine ammeters will equal the total current as nearly
as possible.

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70 ELECTRIC TRANSMISSION §23

The problem, then, of making a proper division of the
load is more diflficult in the case of alteraators than direct-
current machines. The alternators are compelled to run at
the same speed just as if they were actually geared to a com-
mon shaft, and any decrease in the speed of one must be
accompanied by a corresponding decrease of speed in the
other. Now, the governors of steam engines and water-
wheels are designed so that a certain small decrease in speed
is necessary, with increase of load, to make them operate.
For example, suppose a steam engine is carrying a light
load and running at a certain speed. If the load is increased,
the speed must drop a slight amount before the governor
can operate to admit steam sufficient to carry the load, and
the engine continues to run at a slightly lower speed on the
heavy load than it did on the light load. There is therefore
a certain engine speed for each load.

Now, suppose that two alternators are running in parallel
and that each is supplying half the amotmt of power taken
by the system. If the external load is increased, the amoimt
of power supplied to each alternator must also increase, and,
if the load on the machines is to be kept equal, each engine
must increase its power output by an equal amount. We have
just seen that to increase the power output the engine speed
must drop slightly, and as the alternators must always run in
synchronism, it follows that both engines must, for a given
increase in load, drop their speeds an equal amount. In other
words, to secure equal division of load the engines must per-
form in exactly the same way as regards change in speed with
change in load. If one drops its speed more than the other, it
takes the load and the other machine may even be driven as a
synchronous motor. The question, then, of proper division of
load is one that relates more to the engines than to the alter-
nators, and in choosing engines for this kind of work every
effort should be made to have them alike as regards their
change in speed with change in load. The engines may run at
exactly the same speed for a given load, but if their speeds do
not drop by the same amount with increase in load, the out-
put will not divide properly between the machines.

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§23 ELECTRIC TRANSMISSION 71

When machines are belt-driven, great care must be taken
to see that the pulleys are exactly the correct dimensions to
give the speeds required for operating in synchronism;
because, if this is not the case, there will be considerable
belt slippage, and there will also be considerable cross-
current between the two machines.

80. Hunting: ol Alternators* — When alternators are
coupled directly to slow-moving steam engines, diflficulty is
frequently encountered in connection with their parallel
operation. This is specially the case when the alternators
deliver a current of high frequency. The machines surg^e,
or bunt, that is, the speed may fluctuate during each
revolution, thus causing large periodic cross-currents to flow
between the machines and seriously affecting the voltage of
the system. This surging may become so bad as to cause
the machines to fall out of synchronism and render parallel
operation impossible. If rotary converters or synchronous
motors are operated from the alternators, surgings are also
set up in them and the voltage fluctuation and sparking
caused thereby may be so serious as to make satisfactory
operation very difficult to accomplish.

The cause of these surgings has been found in many cases
to be due to periodic variations in the speed of the engine,
and various methods have been tried to suppress them.
The turning effort exerted on the crankpin of a steam engine
is not uniform at all parts of the stroke, the pressure at the
various points depending on the steam distribution in the
cylinder or cylinders, on the position of the crankpin,
angularity of the connecting-rod, etc. The result is, that
while the speed of the engine may remain practically con-
stant so far as the number of revolutions per minute is
concerned, there will be momentary variations in speed
during each revolution. It takes but a small momentary
variation in angular velocity to throw the machines con-
siderably out of phase, especially if the alternator has a large
number of poles. For example, if a direct-connected alter-
nator has 60 poles, the angular distance between centers of

45— i7

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72 ELECTRIC TRANSMISSION §23

poles will be 6°, and this corresponds to a phase diflEerence
of 180®. The periodic variation in the angfular velocity of
the revolving field or armature sets up corresponding varia-
tions in phase difference and results in periodic surges of
current between the machines. This trouble has been inves-
tigated quite fully by Mr. W. L. R. Emmett*, who found
that the energy necessary to maintain these current oscil-
lations was in a niunber of cases supplied from the steam
cylinders of the engines, and that it could be largely pre-
vented by fixing the governor so that it would not respond to

these sudden varia-
tions and admit the
steam necessary to
maintain them. The
governor must, how-
ever, be capable of re-
sponding to changes
in the regular load on
the machine, other-
wise enough power
would not be fur-
nished to the alter-
nator to enable it to
carry its share of the
load. In order to fix
the governor so that
Pio.27 it would respond to

. gradual changes in
the load, but not to momentary oscillations, it was pro-
vided with a dashpot similar to that shown in Fig. 27. This
dashpot was designed by Messrs. H. W. Buck and Harte
Cook. It consists of a cylinder A in which a piston B
moves; two by-passes by b' are provided, and at the end of
each is placed a valve ^ or ^ ordinarily held closed by
springs d, d'. Each valve is provided with a small by-pass
ey e'y and the whole cylinder, including the ports, is filled with

*Transactions of American Institute of Electrical Engineers, Octo-
ber 25, 1901.

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§23 ELECTRIC TRANSMISSION 78

heavy oil. Unless valves c, d are raised, the only passagfe
for the oil, to allow movement of the piston, is through the
small ports, and the piston is therefore practically locked.
A sudden fluctuation in the governor will not move c or dy
but a steady pressure on the piston, due to a prolonged
raising or lowering of the speed, will move them, and the
oscillations of the governor and steam in the cylinders are
thereby damped out, thus suppressing the htmting action of
the alternators.

81. In order to prevent hunting effects, engine builders
have endeavored to secure uniform angular velocity of their
engines. In some cases this is accomplished by the use of
very heavy flywheels, but it is a question whether heavy fly-
wheels are on the whole advisable. Some authorities claim
that the momentum of heavy flywheels tends to maintain the
oscillations, and that it is better to use fairly light fljn^heels
and design the engine so that the turning effort on the shaft
will be nearly uniform. By using two or more engines
coupled to the same shaft with their cranks at the proper
angle to each other, this result can be attained quite closely.
This is readily accomplished by cross-compound engines,
either horizontal or vertical, and both types are largely used
for driving alternators. In the case of the large alternators
of the Manhattan Elevated Railway, New York, each
alternator is driven by four engines, two of which are vertical
and two horizontal. There is a crankpin at each end of the
shaft, and to it is connected one vertical and one horizontal
engine. The cranks are displaced 135° and since the four
cylinders give eight impulses during each revolution, the
turning moment is so uniform that no flywheel other than the
revolving field of the alternator is necessary.

82. Use of Damplnsr Devices* — Another method that
has been used to prevent htmting is to provide special wind-
ings or conductors on the alternator field, so that the currents
set up in them will oppose any shifting action and thus retard
the oscillations. This device has been used much more on
European alternators than on. those built in America.

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ELECTRIC TRANSMISSION

§23

Fig. 28 (a) shows the method of arranging a damper (French
amortissenr) of this kind, due to Rutin and Leblanc. A is
the laminated pole piece of a revolving field alternator and
is provided with the usual exciting coil B^ Near the surface
of the pole piece are a number of slots in which copper bars c
are placed. These bars are connected together at each end
of the pole by means of copper straps, thus forming the bars
into a number of closed circuits similar to the squirrel-cage
armature of an induction motor. As long as the magnetic
flux passing from the pole face into the armature remains
stationary with respect to the pole face, no currents are set
up in the bars. If, however, there is any momentary shift-
ing of the field, heavy currents are set up in the bars, and

Pig. 28

these currents dampen the motion, thus smoothing out any
tendency toward fluctuation. Fig. 28 (d) shows a field con-
struction used by the Westinghouse Company that has some-
what the same effect. Copper bridges A are placed between
the poles; these serve to hold the coils in place and dampen
hunting effects.

83, Hunting sometimes occurs even when the alternators
are driven by prime movers, such as steam or water tur-
bines, that give an absolutely uniform angular velocity. In
this case the effect is due to certain relations between the
properties of the electric circuit, such as its self-induction,
capacity, etc., and the momentum of the moving masses of the
machinery. The result is a cumulative pendulum effect that
may be overcome by changing some of the above properties

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§23 ELECTRIC TRANSMISSION 75

of the circuit or by damping the alternator, synchronous
motors, rotary converters, or other devices on the system.
For example, a change in field excitation will frequently
overcome the difficulty. Fig. 29 shows another arrange-
ment used for preventing hunting of rotary converters and
alternators. The pole piece

is provided with a slot b in
the.center, in which is placed
a heavy copper bar. The
pole is also encircled by a
heavy conductor forming
two local circuits, in which
heavy currents are set up if

there is any shifting of the ^ ^ ^ ^

field. Rotary converters are
also frequently provided
with copper bridges between
the poles, about as shown
in Fig. 28 (^) , to dampen the
hunting. Fig. 30 shows an
anti-hunting device used on
General Electric converters. ^'°- ^

The copper casting a, b, e, / bridges across the pole tips
and is held in place by a bolt passing through a b. By draw-
ing up this bolt, edges ei are forced apart against the pole
tips. The sides cd lie in slots provided in the pole faces.

84. Generally speaking, the practice in America is to
obtain engines that will give a nearly uniform angular
velocity, though damping devices are also used. Damping
devices add to the cost and also slightly lower the efficiency
of the machines to which they are applied. Engine builders
will now guarantee engines not to give a departure from
uniform motion during a revolution that will cause more
than 2i® to 3® of phase displacement of the E. M. F. furnished
by each of the alternators or a total maximum phase displace-
ment of 5° to 6®. If the displacement does not exceed this
amount, the operation should be satisfactory. In America

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ELECTRIC TRANSMISSION

§23

Pig. 80

damping devices are more commonly used on rotary con-
verters than on alternators.

When steam-driven alternators are being synchronized, it
is necessary to have some convenient means of controlling
the engine speed from the switchboard. One way of doing
this is to have a small reversible electric motor attached to
the governor and arranged so that it can vary the tension on

a spring attached to
the governor weights
or vary the position
of a weight on a
lever arm attached to
the governor. This
motor is readily
started, stopped, or
reversed from the switchboard, so that the attendant has the
speed of the engine under control and can make the slight
variations in speed necessary to secure equality of frequency.
Also, this device allows the point of cut-off to be varied when
the engine is in regular operation, thus regulating the amount
of power supplied to the alternator. As explained above,
the current delivered by each alternator when running in
synchronism depends on the amount of power supplied to
the alternator, so that by adjusting the governor, the output
of each machine, as shown by its indicating wattmeter on
the switchboard, can be regulated.

85. Compound -Wound Alternators In Parallel.

Most of the large alternators now installed are of the
revolving field type and are not generally provided with a
compound field winding. For large units it is found that a
carefully designed machine gives sufficiently close voltage
regulation with a plain, separately excited winding, so that
the extra complication of compound field excitation is not
warranted. Where a compound winding is used on the
fields, it is necessary to provide an equalizing connection
somewhat similar to that used for a direct-current machine.
Fig. 31 shows the connections necessary for running two

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§23

ELECTRIC TRANSMISSION

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78 ELECTRIC TRANSMISSION §23

compound-wound, three-phase alternators in parallel, the
connections for the separately excited field being omitted in
order to simplify the diagram. The terminals of the series-
field winding on each machine connect through switches
Ay A to the equalizing wires b, b. An adjustable resistance r
is connected across each field, so that the effect of the
series-coils can be varied to suit the character of the load
on the machines. With the synchronizing connections shown
in the figure, the lamps will be bright at synchronism, though
the lamps could be made dark by simply changing the cross-
connections used with the plug on the machine being syn-
chronized. In this case an ammeter is used in one phase
only, and is all that is necessary to indicate the current,
provided the load is of such a nature that it is not liable to
become unbalanced. In many cases it is customary to use an
ammeter in each line, so that the current in all three phases
will be indicated.

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LINE CONSTRUCTION

INTRODUCTION

1. liine construction may be considered conveniently
under two heads: (a) overhead construction; (^) underground
construction.

For nearly all work in towns and small cities or for cross-
country work, the lines are supported on poles. In cities,
the current is now usually distributed, at least so far as the
central part oiE the cities is concerned, by means of wires or
cables run in underground tubes or ducts. This method is,
of course, much more expensive than the overhead method;
but the large increase in the number of wires used for
different electrical purposes has rendered imderground dis-
tribution in cities almost absolutely necessary.

lilNE CONDUCTORS

2. The line wire is, in the vast majority of cases, of
copper. Aluminum is now coming into use for this purpose,
and in the future it may replace copper for some lines of work.
Iron or steel is seldom used for a line conductor, because its
resistance is too high. There is one case, however, in which
it is largely used as a return conductor, and that is in con-
nection with electric railways, where the current is led back
to the power house through the rails.

COPPER CONBTTCTORS

3. Bare and Insulated Wires* — Line conductors are
usually in the form of copper -wire of round cross-section
whenever the conductor is of moderate size. For conductors

For n0tiot of copyright, tu page imwudiaUly following the title Page

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LINE CONSTRUCTION

§24

of large cross-section, stranded cables are used, made up of a
number of strands of small wire twisted together. This con-
struction makes the conductor flexible and easy to handle.
When these wires or cables are strung in the air, they are
usually insulated by a covering that consists of two or three

Pio. 1

braids of cotton, soaked in a weather-proof compound com-
posed largely of pitch or asphalt. For underground work,
the conductor is first insulated with rubber, or paper soaked in

Pig, 2

compound, and the whole covered with a lead sheath to keep out
moisture. Fig. 1 shows a stranded cable for underground
work provided with an insulating layer of paper and a lead

Fio. 8

sheath. Fig. 2 shows an ordinary triple-braid weather-proof
overhead line wire, and Fig. 3 a weather-proof overhead
cable. When the pressure used on the line is very high, say
10,000 volts or more, bare wires are generally used, because
the ordinary weather-proof insulation is of little or no

Digitized by VjOOQIC

§24 LINE CONSTRUCTION 3

protection against such pressures and only gives a false
appearance of security. The practice for such lines is, there-
fore, to use Bare wire and to insulate it thoroughly by
means of specially designed insulators.

WIRE GAUGES

4. Various standards or -wire ^^auges have been
adopted by diflEerent manufacturers, but the safest and best
way is to express the diameter of a wire in mils, or
thousandths of an inch, and its area of cross-section in circu-
larmils. The American, or Brown & Sharpe, gauge is used
almost exclusively in America in connection with electrical
work, but it is always well to give the diameter of the wire
as well as its gauge number, so as to avoid any possibility
of mistake. When wires or cables larger than the regular
B. & S. sizes are specified, their cross-section is given in
circular mils. Explanations regarding the B. & S. gauge
and the expression of area in circular mils, etc. have already
been given, so it will not be necessary to repeat them here.
As we shall have occasion to refer to the B. & S. wire table
frequently. Table I is repeated here for convenience. This
gives the dimensions, weight, etc. of bare copper wire
according to the B. & S. gauge for both annealed and hard-
drawn wire; most wires and cables are of annealed copper.
The use of hard-drawn copper is confined principally to
trolley wire for street railways and telephone and telegraph
line wires.

5. Table II gives the approximate weights of weather-
proof line wire, such as is used for ordinary outside lines.

6. Table III gives the approximate dimensions of
strandard insulated weather-proof cables for overhead work.
Such cables are always designated by their area of cross-
section in circular mils, and not by gauge number. In
fact, any conductor larger than No. 0000 is usually desig-
nated by its area in circular mils. Cables such as those
given in Table III are extensively used for street-railway
feeders or for any other purpose requiring a large conductor.

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Digitized by VjOOQIC

LINE CONSTRUCTION

§24

ALUMINUM CONDUCTORS

7. Mention has already been made of the fact that
alnminum is being used for electrical conductors, because
this metal can now be sold at a figure low enough to
compete with copper. Its conductivity is only about 60 per
cent, that of copper, so that for a conductor of the same
resistance a larger cross-section is required. Aluminum is,
however, so much lighter than copper that the larger cross-
section can be used and still compete with the latter metal,
although the cost per pound of the aluminum is considerably

TABI.E n
APPROXIMATE TVEIGHTS OF WBATHER-PROOF WIRE

(Amert'can Electrical H^orks)

Triple-Braided Insulation

Size

Feet per
Pound

Pounds per
1,000 Feet

Pounds
per Mile

Canying Capac-
ity, Amperes,
National Board
Fire Underwriters

0000

1.34

742

3,920

312

000

1.64

609

3,215

262

2.05

487

2,570

220

2.59

386

2,040

185

3.25

308

1,625

156

4.10

244

1,289

131

5.15

194

1,025

•6.26

160

845

7.46

134

710

9.00

III

585

13-00

385

20.00

265

29.00

182

38.00

137

48.00

113

67.00

Digitized by VjOOQIC

§24

LINE CONSTRUCTION

TABUS 11— {Continued)
Double-Braided Insulation

Size

Feet per
Pound

Pounds per
1,000 Feet

Pounds
per Mile

Carrying Capac-
ity, Amperes,
National Board
Fire Underwriters

0000

1.40

711

3,754

312

000

1.75

570

3,010

262

2.29

436

2,300

220

2.81

355

1,875

185

3.56

281

1,482

156

4.49

223

1,175

131

5.45

184

969

6.82

147

774

9.10

580

10.35

510

15.52

340

22.00

237

40.00

132

56.00

76.00

100.00

higher. Line -construction work is somewhat more diffi-
cult with aluminum than with copper; joints are more
difficult to make and there is greater liability of the spans
breaking. Table IV gives the properties of aluminum wire
of the grades made by the Pittsburg Reduction Company
and Table V gives the resistance. The values in these tables
are taken from a pamphlet issued by the above company. A
comparison of some of the properties of aluminum and copper
is given in Table VI.

Digitized by VjOOQIC

LINE CONSTRUCTION

§24

TABI.E m
STANDARD WEATHER-PROOF FEED-WIRE

(liaedliHjr's)

Circular Mils

5 -g

'2
0

Weights
Pounds

Approximate Length
on Reels

Feet

Carrying Capacity,

National Board

Fire Underwriters

1,000 Feet

Mile

1,000,000

3,550

18,744

800

1,000

900,000

3,215

16,975

800

920

800,000

2,880

15,206

850

840

750,000

2,713

14,325

850

700,000

i-h

2,545

13,438

900

760

650,000

2,378

12,556

900

600,000

2,210

11,668

1,000

680

550,000

2,043

10,787

1,200

500,000

1,875

9,900

1,320

590

450,000

1,703

8,992

1,400

400,000

1,530

8,078

1,450

500

350,000

1,358

7,170

1,500

300,000

1,185

6,257

1,600

400

250,000

1,012

5,343

1,600

Digitized by VjOOQIC

§24

LINE CONSTRUCTION

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LINE CONSTRUCTION

§24

TABLE V

TABLE OF KBSISXANCES OF PURE

ALUMINUM WIRE*

Resistance at 75® P.

ct •

Ohms

Feet

Ohms

Ohms
ix)ooPeet

MUe

oEm

per
Potrnd

oooo

.08177

.43172

12,229.8

.00042714

ooo

.10310

.54440

9,699.00

.00067022

.13001

.68645

7,692.00

.0010812

.16385

.86515

6,245.40

.0016739

.20672

1.09150

* 4,637.35

.0027272

.26077

1.37637

3,836.22

.0043441

.32872

1.73570

3,036.12

.0069057

.41448

2.18850

2,412.60

.010977

.52268

2.75970

1,913.22

.017456

.65910

3.48020

I. 517.22

.027758

.83118

4.38850

1,203.12

.044138

1.06802

5.53550

964.180

.070179

1. 32135

6.97670

756.780

•.II156

1.66667

8.80000

600.000

.17467

2.10120

11.0947

475.908

.28211

2.64970

13.9900

377.412

.44856

3.34120

17.6420

299.298

.71478

4.31800

22.8000

231.582

I. 1623

5.19170

27.4620

192.612

1.7600

6.69850

35.3680

149.286

2.8667

8.44720

44.6020

118.380

4.5588

10.6518

56.2420

93.8820

7.2490

13.8148

72.9420

72.3840

12.192

-20

16.9380

89.4300

59.0406

.18.328

21.3580

112.767

46.8222

29.142

26.9200

142.138

37.1466

46.316 •

33.9620

179.320

29.4522

73.686

42.8250

226.120

23.3508

117.17

54.0000

285.120

18.5184

186.28

68.1130

359.650

14.6814

296.32

85.8650

453.370

11.6460

485.56 .

108.277

571.700 .

92358

749.02

136.535

720.900

7 3242

1,191.0

172.170

908.980

5.8087

1.893.9

212.120

1,119.98

4.7144

2,941.5

273970

1,445.45

3.6528

4,788.9

345.130

1,822.30

2.8974

7,610.7

435.380

2,298.80

2.2969

12,109.

548.920

2,898.20

1. 8218

19,251.

692.070

3,65420

1.4449

30,600.

872.930

4,f)09.20

1. 1456

48,661.

1,100.62

5,811.20

.90S6

76.658.

1,387.47

7.32580

.7207

121,881.

1.749.50

9,236^80

.5716

193.835.

*CalcuIated on the basis of Mattbiessen's standard.

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§24

LINE CONSTRUCTION

TABI.E VI
COMPARISON OF PROPERTIES OP COPPER AND ALUMINUM

Conductivity (for equal sizes) . . .

Weicrht (for equal sizes)

Weight (for equal length and re-
sistance)

Price, aluminum 29c.; copper i6c.
(bare line wire)

Price (equal resistance and length,
bare line wire)

Temperature coefficient, degree F.

Resistance of mil-foot (20° C.) . .

Specific gravity

Breaking strength (equal sizes) . .

Tensile strength (poimds per square
inch, hard drawn)

Coefficient of expansion, degree F.

Alamintim

Copper

.54 to .63
.33

I
I

.48

1.81

.868
.002138

18.73
2.5 to 2.68

.002155

10.05

8.89 to 8.93

40,000
.0000231

60,000
.0000093

IRON WIRE

8. Iron wire is used largely for telegraph and telephone
work, but it is seldom employed in connection with electric
transmission because of its high resistance. The approxi-
mate value of the resistance per mile of a good quality of
iron wire may be determined by the formula

360,000

R =

(1)

where d = diameter of wire in mils.

9. For steel wire, which is often used in place of iron
wire, this formula becomes approximately

R = 470,000 (2)

The various grades of iron wire on the market are termed
"Extra Best Best," *'Best Best,** and **Best*'; the resistances
of the different grades are shown in Table VII.

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LINE CONSTRUCTION

§24

TABIiE Vn
DIMENSIONS AND RESISTANCE OF IRON WIRE

flQ

. a

t
<

Welsrht
Pounds

Breakinff
Strength

Pounds

Resistance per

MUe

1. 000
Feet

I MUe

Iron

Steel

B. B. B.

B. B.

Steel

340

115,600

304.0

1.607

4,821

9,079

2.93

3.42

•4.05

300

90,000

237.0

1,251

3,753

7,068

3.76

4.40

5.20

284

80,656

212.0

1,121

3,363

6,335

4.19

4.91

5.80

259

67,081

177.0

932

2,796

5.268

5.04

5.90

6.97

238

56,644

149.0

787

2,361

4,449

5.97

6.99

8.26

220

48,400

127.0

673

2,019

3,801

4.99

8.18

9.66

203

41,209

109.0

573

1,719

3.237

8.21

9.60

11.35

180

32,400

85.0

450

1,350

2,545

10.44

12.21

14.43

165

27,225

72.0

378

1,134

2,138

12.42

14.53

17.18

148

21,904

58.0

305

9^5

1,720

15.44

18.06

21.35

134

17,956

47.0

250

750

1,410

18.83

22.04

26.04

120

14,400

38.0

200

600

1,131

2348

27.48

32.47

109

11,881

31.0

165

495

933

28.46

33.30

39.36

9.025

24.0

125

375

709

37.47

43.85

51.82

6,889

18.0

288

541

29.08

57.44

67.88

5,184

13.7

216

407

65.23

76.33

90.21

4,225

II. I

• 59

177

332

80.03

93.66

110.70

3.364

8.9

141

264

100.50

120.40

139.00

2,401

6.3

189

140.80

164.80

194.80

GERMAN-SILVER WIRE

10. German-silver ivlre is used principally in resist-
ance boxes or electrical instruments where a high resistance
is required. The resistance of this wire varies greatly
according to the materials and methods of manufacture used.
It is an alloy of copper, nickel, and zinc, and has a resistance
anywhere from 18 to 28 times that of copper. Its resistance
changes only to a small extent with changes in temperature,
a feature of value in connection with rheostats and resistance
boxes.

Digitized by VjOOQIC

§24

LINE CONSTRUCTION

Table VIII gives some of the properties of German-silver
wire containing 18 or 30 per cent, of nickel.

TABIiE Tin
GERMAN-SILVER WIRE

{Roebline's)

Resistance per i.ooo Feet

Maximum Cur-
rent Carrying
Capacity in

Number

IntematioDal Ohms

B.&S. Gauge

Amperes

i8-Per-Cent. Wire

30-Per-Cent. Wire

i8-Per-CeBt.
Wire

7.20

II. 21

9.12

14.18

11-54

17.95

14.55

22.63

18.18

28.28

8.5

22.84

35.53

5.4

28.81

44.82

4.6

36.48

56.75

3.8

46.17

71.82

3.2

58.21

90.55

2.7

. 72.72

113. 12

2.3

93.40

145.29

1.9

118.20

183.87

1.65

145.94

227.02

1.21

184.68

287.28

.99

232.92

362.32

.88

295.38

459.48

.66

370.26

575.96

.55

468.18

728.28

.488

590.22

918.12

.434

748.08

1, 1 63.68

.385

937.98

1,459.08

.343

1,191.24

1,853.04

1,481.22

2,304.12

1,891.8

2,942.8

2,388.6

3.7I5-6

2.955.6

4.597.6

3,751.2

5,835.2

4.764.6

7.411.6

6.031.8

9,382.8

7,565.4

1 1.768.4

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14 LINE CONSTRUCTION §24

OVERHEAD CONSTRUCTION

POIiES

11. Selection of Poles. — The poles used to the grreat
est extent in this country are of the following kinds of wood:
Norway pine, chestnut, cypress, and white cedar. The
average lives of these, under average conditions, are placed
by good authority at the following values: Norway pine,
6 years; chestnut, 15 years; cypress, 12 years, white cedar,
10 years. Cedar poles are undoubtedly used to the greatest
extent. Considering their strength, they are light in weight,
and, by some authorities, are considered the most durable,
when set in the ground, of any American wood suitable
for pole purposes. In some of the Western States, Califor-
nia redwood is used for poles.

12. Sizes of Poles. — The best lines in this country use
no poles having tops less than 22 inches in circumference.
If the poles taper at the usual rate, the specification that a
pole shall have a top 22 inches in circumference, or approxi-
mately 7 inches in diameter, is usually sufficient, for the
diameter at the butt will then be approximately correct, no
matter what may be the length of the pole. When a pole line
has to carry but a few small wires, it is not necessary to have
them as large as 7 inches at the top, and poles with a 5-inch
top will answer every purpose. For long-distance transmis-
sion work, only the most substantial line construction is
allowable, because every precaution must be taken to mak^
the service continuous. Long transmission lines usually
have to carry heavy wires, and moreover they are often in
very exposed localities; for this class of work, therefore,
specially heavy poles are used. The length of poles used in
any given case is fixed by several considerations. It will

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§24

LINE CONSTRUCTION

depend to some extent on the number of cross-arms to be
accommodated, but more frequently the length is detennined
by the location of the pole. In any given transmission line
it is necessary to use a number of different pole lengths and
select the poles so that the tops will be graded, thus avoiding
ups and downs in the wire as much as possible. A poorly
graded line requires a greater length of wire than a well
graded one, and this is objectionable not only on account of the
extra cost of the wire, but also because of the larger line loss
due to the larger resistance. Table IX shows the size of
poles used on the Bay Counties high-tension transmission

TABIiB rX
DIMENSIONS OF POL.E8

Height

Diameter of Top

Diameter of Butt

Depth in Ground

Feet

Inches

Feet

line in California*. Where angles occur in the line, the poles
are set 1 foot deeper than shown by the figures in the last
column of the table.

13. Spacing of Poles. — Practice varies as to the spa-
cing of poles. Of course, the number and sizes of the wires
to be carried are the most important considerations in deter-
mining this point, but the climatic conditions, especially with
regard to heavy wind and sleet storms, should also be
considered. In general, it may be said that the best lines
carrying a moderate number of wires use 40 poles to the
mile, while for exceptionally heavy lines, the use of 52 poles
to the mile, or 1 pole every 100 feet, is not uncommon practice.

* Journal of Electricity, Power, and Gas, Vol. XI, No. 8.

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16 LINE CONSTRUCTION . §24

As a general rule, which it is safe to follow in the majority
of cases, 35 or 40 poles to the mile should be used. For city
work, the poles should be set on an average not farther
apart than 125 feet.

CROSS- ARMS
14. The cross-arms should be made of well-seasoned,
straight-grained Norway pine, yellow pine, or creosoted
white pine. Cross-arms are made in standard sizes, the

Fio. 4

length of the arm depending on the number of pins it is
intended to hold. The standard cross-arm is Si inches by
4i inches, and varies in length usually from 3 to 8 feet.
They are usually bored for li-inch pins and provided with
holes for two i-inch bolts. The arms are generally braced
by flat iron braces, about I4 inches wide by i to t inch thick.
These braces are shown in Fig. 4, which gives a view of
an ordinary pole top provided with two 4-pin cross-arms.
This pole top represents the style of construction suitable
for fairly light work, such as is used for local light and

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§24

LINE CONSTRUCTION

power distribution. For long transmission lines, heavier
cross-arms are used. For example, those used by the
Standard Company, of California, on a line designed to
handle current at 60,000 volts, are 5i inches by 6i inches,
and the holes for the pins are 42 inches apart, this wide
distance between the wires being necessary on account of
the high voltage. The older Niagara line used cross-arms
4 inches by 6 inches, and the later line 5 inches by 6 inches.

15. Fig. 5 shows the pole top used on the first Niagara
transmission line. It was designed to accommodate twelve

Pio. 6

transmission wires, the insulators being placed side by side
on the cross-arms as shown in the left-hand half of the
figure. It was foimd that this arrangement did not work
well because it was an easy matter to start short circuits
between the wires, and the arc thus started traveled along
the line wires until the power was shut off. By adopting
the triangular arrangement shown at the right, the distance
between the wires was doubled and all three wires made
equidistant from each other. The apex of the triangle formed
by the wires was placed downwards, as this arrangement

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18 LINE CONSTRUCTION §24

makes it more difficult to lodge sticks or wires across
the circuit than if the single wire is placed on the top arm
with the other two beneath it, though the latter arrangement
is used quite often. The Niagara line is designed to oper-
ate at 20,000 volts. The supports a, a at each end of the
cross-arms were intended to hold barb wire that was
grounded at regular intervals in order to conduct off light-
ning discharges. The barb wire was also intended to act to
a certain extent as a guard wire to prevent articles from
falling on the line. It was found, however, that sleet and snow
caused these guard wires to break and fall across the lines,

Fig. 6 Fio. 7

thus giving rise to so much trouble that they were finally
removed. Barb wire is nevertheless used successfully in con-
nection with a number of transmission plants, and affords an
efficient protection against lightning, but it is necessary to use
wire that is heavy enough to stand the strains put on it.
Ordinary light barb wire as used for fences is not heavy enough
for work where it has only one support in, say, every 100 feet,
as is the case on a pole line. Another method that is some-
times used for arranging two three-phase circuits is to use three
cross-arms with two wires on each cross-arm, the pins being so
placed that the wires come at the corners of a regular hexagon.

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§24 LINE CONSTRUCTION 19

PINS

16. One style of pin by which insulators are mounted on
cross-arms is shown in Fig. 6. This shows the ordinary
pin used for light lines; pins used for heavy long-distance
lines are considerably larger and stronger. They may be
made of locust, chestnut, or oak (the woods being preferred
in the order named), and are turned with a coarse thread
on the end on which the insulator is to be seciured; the
shank K is li inches in diameter.

• — 1

Fig. 8 Pio. 9

The pin should be secured in the hole by driving a nail
through the arm and the shank. This renders it difficult to
extract the shank of the pin in case a new one is required;
but, on the other hand, it prevents the pin pulling out, which
sometimes occurs when this precaution is not taken. For
heavy lines, pins having an iron bolt passing through them
are sometimes used. Fig. 7 shows a pin of this kind,
designed by F. Locke, with a heavy insulator for carrying a
cable in the groove a.

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20 LINE CONSTRUCTION §24

In the case of high-tension, long-distance lines, exception-
ally strong pins should be used. These are made of wood,
because with high pressures any metal is objectionable near
the insulator. Fig. 8 shows the style of pin used by the
Standard Company previously referred to. These pins are
made of blue gum wood {Eucalyptus), specially treated with
linseed oil to prevent them from absorbing moisture. This
pin is also shown in Fig. 14 in connection with the insulator
that it supports. Fig. 9 (a) and {b) shows two styles of
pin used on the Niagara transmission lines; {b) is the old-
style pin, which was found to be too weak; . {a) shows the
heavier pin used on the later line. Note that in (a) the hole
for the pin does not pass completely through the cross-arm.
About 1 inch of wood is left at the bottom, as this is found
to greatly strengthen the cross-arm.

Pio. 10 Pio. u

17. Insulators in this country are usually made of glass,
while in Europe porcelain is more commonly used. Porcelain,
when new, is a better insulator than glass; but it is more
costly, and under the action of cold the glazed surface
becomes cracked. When this happens, the moisture soaks
into the interior structure, and its insulating quality is greatly
impaired. Tests recently made have shown that when newly
put up, the insulation resistance of porcelain insulators is
from 4 to 8 times better than glass, but that, along railroads
and in cities, smoke forms a thin film on each material, so
that at the end of a few months their insulating properties
are nearly alike. On country roads, away from railroad

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§24 LINE CONSTRUCTION 21

tracks, the porcelain insulators maintain a higher insulation
than the glass during rain storms, but in fine weather it is
not so high. Porcelain has an advantage over glass in that
it is not so brittle, and therefore is less likely to break when
subjected to mechanical shocks. It does not condense and
retain on its surface a thin film of moisture so readily as
glass, i. e., it is less hygroscopic. On the other hand, glass
insulators are not subject to such an extent as porcelain to
the formation of cocoons and cobwebs under them, the
transparency of the glass serving to allow sufficient light to
pass through the insulator to render it an undesirable abode
for spiders and worms. As cocoons, cobwebs, etc. serve to
lower the insulation of the line to a great extent, this is an
advantage that, in this country, it is not well to overlook.

Pig. 12 Pio. IS

18. Types of Insulators. — For ordinary work with
moderate pressures, glass insulators are used. The style of
insulator will depend to some extent on the size of wire to be
supported. Wires smaller than No. 6 or 8 B. & S. are seldom
used for power transmission lines; hence, the glass insulators,
as a rule, must be heavier than the kind used for telegraph or
telephone work. Fig. 10 shows an insulator, known as the D. G.
(deep groove), that is well adapted for ordinary lines. This
insulator is so called to distinguish it from those with smaller
grooves, such as are used for telephone or telegraph work.
It is provided with two petticoats, or flanges, a, b over which
leakage must take place before the current can leak from the
wire to the pin. The use of a number of petticoats increases
the leakage distance and provides a high insulation; insula-
tors used on high-tension lines are provided with several

Digitized by VjOOQIC

LINE CONSTRUCTION

§24

petticoats. When heavy cables are used, it is customary
to carry them on especially heavy insulators and to tie
down the cable on top of the insulator instead of tying it to
the side. Fig. 7 shows a common type of such insulator;
the cable rests in the groove a and is held in place by a tie-
wire twisted around the cable and passing under the ears
at b, c. Good quality glass insulators, such as those just
described, may be used for any lines where the potential is not

over 2,000 or 3,000 volts;
for higher pressures, it is
necessary to use a larger
insulator giving a higher
degree of insulation.
Fig. 11 shows a Locke
insulator of glass that is
suitable for any pressure
up to 5,000 volts. This
insulator is Ai inches in
diameter, and, it will be
noted, is provided with
three petticoats, thus giv-
ing a long leakage dis-
- tance from the wire to the
pin. Fig. 12 shows a still
larger insulator; this one
is suitable for pressures
up to 25,000 volts and is
bi inches in diameter.
For high pressures, por-
celain insulators have been largely used; as yet there does
not seem to be any settled opinion as to just which is the
better, glass or porcelain, for this kind of work, and on some
lines using very high pressures the insulators are made partly
of porcelain and partly of glass. Fig. 13 shows a type of
porcelain insulator used for one of the Niagara-Buffalo trans-
mission lines. These insulators are elliptical, or helmet,
shaped and have an eave, or ridge, a on each side, the
object of which is to run off the water to the end of the

Fio. 14

Digitized by VjOOQIC

§24 LINE CONSTRUCTION 23

insulator, where it will drop clear of the cross-arm. Fig. 9 (a)
shows a section of the later type of insulator used on the
Niagara lines, and Fig. 14 shows a style that is used on high-
tension lines in California that operate at pressures as high
as 40,000 to 60,000 volts; in fact, lines equipped with these
insulators have been operated experimentally at 80,000 volts.
This insulator is made in two parts, the upper part being of
porcelain and the lower of glass. The parts are cemented
together by a mixture of sulphur and sharp sand, and the
upper part is made of porcelain because moisture does not
cling to it as readily as to glass. Glass ofiEers a greater
resistance to puncture than porcelain, so that by combining
the two materials a very
efficient insulator is ob-
tained, and the cost is
also reduced materially.
The lower part of the
pin is covered by a por-
celain sleeve that pro-
tects the pin from any
arc that might tend to
strike from the eave of
the insulator, and it also
protects the pin from

the weather. The upper Pio. 15

part of the insulator is

provided with a ridge around the edge and a projecting lip at
one side, so that rain falling on the insulator drips clear of the
cross-arm. These insulators are subjected to a test pressure
of 120,000 volts for a period of 5 minutes in order to detect
any defective insulators before they are put up on the line.

TYING, SPLICING, ETC.

19. Tying. — Fig. 15 shows the method of tying that is
commonly used for small insulators. The tie-wire a is from
12 to 16 inches in length and should be insulated to the
same extent as the wire to be tied. The line wire is laid in

Digitized by VjOOQIC

24 LINE CONSTRUCTION §24

the groove of the insulator, after which the two ends of the
tie-wire, which have been passed half way around the insulator,

are wrapped tightly around
the wire. Some linemen pre-
fer to wrap one end of the
tie-wire over and the other end
under the line wire. Fig. 16
shows a method of tying used
where the wire lies on top of the
„ , insulator as with the Niagara

PlO. 16 *•

type. Fig. 17 shows the method
of tying to the insulator shown in Fig. 14. In this case a
No. 4 aluminum tie-wire is used to tie the aluminum cable.

20. Splicing. — The American wire joint shown in
Fig. 18 is generally used for splicing solid wires. The
wires are placed side by side and each end wound around
the other. All joints should
be soldered. The rules of the
National Board of Fire Under-
writers require that all line
joints shall be mechanically
and electrically perfect before
being soldered; i. e., solder
should not be depended on to
make the joints strong mechan-
ically or efficient as an elec-
trical conductor. In other
words, soldering should always
be done simply as a safeguard
against any diminution in the
electrical conductivity of the ^^^ ^^

joint. Large copper cables are

joined either by weaving the strands together and soldering,
or by using a copper sleeve into which the ends of the cable
are fastened.

Aluminum wires and cables are very often joined by
means of a mechanical coupling, as aluminum is not easily

Digitized by VjOOQIC

§24 LINE CONSTRUCTION 25

soldered. Fig. 19 shows an aluminum mechanical joint used
on a number of California lines. The cable passes through
the sleeves a, a\ which are provided with right- and left-
handed threads, so that they can be drawn tightly together
by the threaded sleeve b. The ends of the cable are first
sawed off square, and after they have been passed through
the sleeves, about 1 inch of each cable strand is bent back on
itself, and the bunch so formed is forced into the conical part

Pio. 18

of the sleeve. A small tapered aluminum plug is then driven
into the center, thus wedging the strands firmly, after which
the ends are securely screwed together. Another method of
using this joint is to turn back on itself about li inches of
the core wire of the cable, and after the strands have been
forced into place and the joint screwed up tight, the space
between the wires is filled with solder. In this case the
turned-back wire takes the place of the aluminum wedge and
spreads out the cable so that it is impossible for it to pull

Pig. 19

through after the joint is filled with solder. Either method
makes a very strong joint of which the resistance is less than a
corresponding length of the cable. Aluminum wires are fre-
quently joined by using a long aluminum sleeve or tube having
an elliptical cross-section. This sleeve fits the wires snugly
when they are slid into it side by side, and after they are in place
they are twisted together. This is a good method for splicing
solid wires; for stranded cables a sleeve joint is to be preferred.

45—19

Digitized by VjOOQIC

26 LINE CONSTRUCTION §24

21. Stringrtngr Aluminum Wire. — Owing to the pecu-
liar physical properties of aluminum wire, special care has
to be taken in stringing it; otherwise, breaks in the line
will be frequent. Slight impurities in aluminum, wire affect
both its mechanical and electrical properties to a marked
extent. Its coefficient of expansion with increase in tem-
perature is high, and if the stress on the wire is as high as
14,000 to 17,000 pounds per square inch, the wire stretches
and takes a permanent set. In stringing the wire, it is
therefore important to allow sufficient sag, in accordance
with the temperature, so that when the wire contracts it will
not be unduly strained. Neglect to do this has resulted in
numerous breaks in some of the line wires that have been
erected. An aluminum line in warm weather looks as if it
had too much sag, but the contraction is so large with
decrease in temperature that this slack is very largely taken
up in cold weather. Table X, given by the Pittsburg
Reduction Company, shows the deflection at the center of
the span that should be allowed for various spans together
with the tension under which the wire should be put up.

In this table X = deflection in inches at center of span;
5 = factor by which weight of wire per foot is multiplied to
obtain tension.

ExAMPLB. — Suppose a No. 4 aluminum wire is strung on poles 150
feet apart; what sag should be allowed at the center, if the temperature
at the time the wire is strung is 30° F.?

Solution. — Opposite the span 150, and under the column for 30°,

we find that the deflection X should be 24 in. The weight of No. 4

200 9
aluminum wire per mile is 200.9 lb., or the weight per foot is ,. '

= .038 lb. Hence, the tension will be A^ X .038 = 1,390 X .038
= 52.8 lb. Ans.

22. In stringing the wire it is customary to pull up a
number of spans at a time. The deflection is measured by
hanging a target on the wire close to the insulator at each
end of the span. One form of target consists of an iron
strip with cross-marks of different colors corresponding to
different deflections. This strip is hung from the wire by

Digitized by VjOOQIC

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Digitized by VjOOQIC

28 LINE CONSTRUCTION §24

means of a hook, and when the lowest point of wire comes
in line with the point corresponding: to the deflection called
for by the temperature at which the wire is stnmg, the line
is tied to the insulator. The correct deflection is easily

III" uim'i^sJ^^

Pio. 20

determined by the lineman sighting from one target to the
other while the wire is being pulled up (see Fig. 20). Each
line foreman is provided with a thermometer and table of
deflections. These refined methods are not necessary in
connection with the stringing of copper wire, and if the cost
of copper and aluminum were equal, copper would doubtless

be used on account of its
superior mechanical qual-
ities. However, in many
cases quite a large saving
can be effected on long
lines by using aluminum,
and this accounts for its
use in connection with this
kind of work. Aluminum
"?o> ^ ' has not as yet been used

Fio. 21 to any great extent for

underground work. The
greater cross-section for a given conductivity is here a
decided objection, because it would for a given current
capacity make the cables considerably larger than those
using copper, and this in turn would call for a larger amount
of insulating material. With bare overhead lines these
objections have little or no weight.

23. Transposition of Transmission Ijlnes. — ^When

a number of alternating-current transmission lines are run
side by side, the alternating magnetic field set up by the
currents in one line may set up E. M. F.'s in the other lines,

ZIXZL

—^

Digitized by VjOOQIC

§24 LINE CONSTRUCTION 29

thus causing unbalancing of the voltage and affecting the
line drop. This disturbing action can be avoided by trans-
posing or spiraling the wires so that the effect produced on
one section of the line will be exactly counterbalanced by
that produced in another. The most perfect example of
spiraling is found in a cable where the conductors that make
up the circuit are twisted
together and the lines
make a complete spiral
every few inches. Such
a cable has practically no
inductive effect on a neigh-
boring cable. Of course,
in overhead transmission
work, transpositions are
not made very numerous
because they make the
wires harder to trace up in
case of trouble and may,
on high-pressure work,
tend to promote crosses.
In fact, some lines that
work satisfactorily are not
transposed at all. The
Niagara lines are trans-
posed in six sections be-
tween Niagara Falls and
Buffalo, about 23 miles.
Practice seems to differ
greatly with regard to the
frequency with which
high-pressure lines should ^^ 22

be spiraled. In some.cases

they are not spiraled at all; in other cases they are spiraled
every 2 or 3 miles. Telephone lines, if strung on the same
poles with transmission lines should be transposed every
fourth or fifth pole, otherwise the telephones may be so noisy
as to render conversation very difficult. Fig. 21 (a) shows

Digitized by VjOOQIC

LINE CONSTRUCTION

§24

the transposition of a single-phase line; (d) sl two-phase line,
and (c) a three-phase line. Fig. 22 shows a transposition on
a high-tension, three-phase line, each wire being shifted
around one pin, or one-third of a turn. Where transpositions

#M9

4000

9500

' —

30O0

Loss on C/rcwt m//j fV/ros
0/ Different Distances,
fre<fuenqy 60. Slotted Armature,
mston Wattmeter.
Wires 15. 22, 35artd 52 incties apart

2SOO

9000

ISOO

1000

500

/■

\ '-^

to M 28
Thoftsands

J2 SO 40 44

of Voite,

59 50 eo

Pig. 23

are made in this way, it is advisable to place the pins on the
cross-arms of each pole a little farther apart than the
standard distance, so that the lines will not come too close
together where they pass each other at the center of the span.

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§24 LINE CONSTRUCTION 81

24. lieakagre on Hi^li-Tension lilnes. — On a hig^h-
tension line there is always some loss due to leakage,
although if the lines be well separated and carefully insu-
lated, this loss may be kept within reasonable limits. The
leakage takes place between the wires either directly
through the intervening air or over the insulators. When
the pressure is raised to a high amount, a brush discharge
takes place between the wires and the loss due to this dis-
charge may be considerable, if the wires are not well
separated. The curves in Fig. 23 show the results of some
tests made by Mr. R. D. Mershon* to determine the relation
between the loss, the pressure, and the distance between
wires. These tests were made on a line about 2i miles in
length. It is seen that there is a certain pressure, for each
distance between wires, beyond which the loss increases
very rapidly and that the nearer the wires are together, the
lower the pressure at which the curves begin to rise rapidly.
The loss by leakage at the insulators, of course, depends to
a considerable extent on the design of the insulator, and also
on its condition, i. e., whether wet or dry. It is difficult,
therefore, to state very definitely what this loss is, but a
number of measurements show that it is in the neighbor-
hood of 2 watts per insulator for lines operated at 25,000
volts, and does not exceed 4 watts with a pressmre as high
as 44,000 volts.

^Transactions American Institute of Electrical Engineers, Vol. XV.

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32 LINE CONSTRUCTION §24

UNDERGROUND CONSTRUCTION

25. In cities, it is necessary to place the wires under-
gfround, especially in the business districts. The best way
to do this is to provide a regfular tunnel, or subway, in which
the various wires, or cables, can be placed and which will be
large enough to allow a man to walk through for inspection
or repair. This method is, however, very expensive and can
only be used in a few very large cities. Another method is
to use conduits through which to run the cables. These con-
duits usually consist of tubes of some kind that are buried
in the ground and thus provide ducts into which the cables
may be drawn. The ducts terminate in manholes usually
placed at street intersections, by which access may be had to
the cables and from which they may be drawn into or out of
the ducts. A third method and one that has been largely
used in cities for distributing current for lighting purposes,
is to bury tubes containing insulated conductors in the
ground. In this system the conductors cannot be withdrawn,
as in the conduit system, and there is a separate tube for
each set of conductors. The Edison tube system belongs to
this variety, and a very large amount of lighting and power
distribution on the three-wire, low-pressure system has been
carried out by using underground conductors of this kind.

CONDUITS

26. A large variety of conduits are in use, and it has
not been definitely settled as yet just which type is the best;
but the following will serve to give an idea as to some of
the more common forms that have stood the test of actual
work and are in extended use.

27. Creosoted-Wood Conduit. — A form of conduit
that was at one time largely used is composed of sections

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§24

LINE CONSTRUCTION

of wooden tubing, the fiber of the wood being impregnated
with creosote, in order to prevent its decay. This form of
conduit is commonly known as pump-lo^ conduit. A
section of this conduit is shown in Fig. 24; the ends are
doweled in order to preserve the proper alinement in joining.
These sections are usually 8 feet
in length, and have circular holes
through their centers from li to
3 inches in diameter, according
to the size of cable to be drawn
in. The external cross-section is
square and 4i inches on the side, in the case of a tube
having a 3-inch internal diameter. Such a conduit as this,
if properly impregnated with creosote, will probably have
a life of from 15 to 20 years, and perhaps much longer,
this point being one concerning which there is considerable
argument and which, probably, time alone will decide. In

PiO.24

Pig. 25

some cases, difficulty has been experienced with creosoted-
wood conduits on account of the creosote attacking the lead
covering of the cables.

28. Cement-Ijliiod Pipe Conduit. — This conduit is
made by the National Conduit and Cable Company. The
sections shown in Fig. 25 are usually 8 feet long and are

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34 LINE CONSTRUCTION §24

made as follows: A tube is made of thin wrought iron, No. 26
B. W. G., .018 inch thick, and securely held by rivets 2 inches
apart. The tube is then lined With a wall of Rosendale

cement f inch thick, the inner sur-
face of which is polished while
drying, so as to form a perfectly
smooth tube. This tubing comes
in three sizes, each having a
length of 8 feet and internal
diameters of 2, 2J, and 3 inches,
the latter being the standard size.
Each end is provided with a cast-
iron beveled socket joint, by the
use of which perfect alinement
may be obtained by merely but-
ting the ends together. These
beveled socket joints also allow
of slight bends being made in the
line of conduit as it is being laid.

29. Vltrlflcd-Clay or
Terra-Co tta Conduit. — A form
of conduit that is probably used
in good construction work to a greater extent than any
other is made of vitrified clay. This material has the
advantage of being abso-
lutely proof against all
chemical action, and unless
destroyed by mechanical
means will last for ages.
Besides this, its insulating
properties are high and
it is comparatively cheap
and easily laid.

Clay, or terra-cotta con-
duits are made m two gen-
eral forms — multiple duct and single duct. Of the former
type the most common is the 4-duct, two sections of which

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§24 . LINE CONSTRUCTION 35

are shown in cross-section in Fig. 26. They are also made
with 2, 3, 4, 6, and 9 ducts.

30. The form of clay conduits now most commonly used
is the single duct shown in Fig. 27; this is usually made
in 18-inch lengths, has an internal diameter of from 3 to 34
inches, and is 4i inches square outside. This duct has a

Pio. 28

great advantage over the multiple-duct sections in the greater
ease of handling and also in the fact that it is much less liable
to become warped or crooked in the process of burning during
its manufacture than the larger and more complicated forms.
Like the cement-lined pipe, it is laid on a bed of concrete.

Fig. 29

cemented together with mortar, and enclosed on all sides and
on top by concrete. In laying, a wooden mandrel, such as is
shown in Fig. 28, 3 inches in diameter and about 80 inches
in length, is used. At one end is provided an eye a, which

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36 LINE CONSTRUCTION §24

may be engaged by a hook, in order to draw it through the
conduit, while at the other end is secured a rubber gasket b
having a diameter slightly larger than that of the interior
of the duct. One of these mandrels is placed in each duct
when the work of laying is begun. As the work progresses,
the mandrel is drawn along through the duct by the workmen,

Pio.80

by means of an iron hook at the end of a rod about 3 feet
long, the method of doing this being shown in Fig. 29.
By this means, the formation of shoulders on the inner walls
of the ducts at the joints is prevented, and any dirt that
may have dropped into the duct is also removed. The
cylindrical part of the mandrel insures good alinement of

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§24 LINE CONSTRUCTION 37

the ducts, thus securing a perfect tube from manhole to
manhole.

31. Fig. 29 illustrates the method of laying this con-
duit, and shows how the joints should be broken in the
various layers so as to insure a maximum lateral strength
to the structure. All conduits should be laid to such grades
that there will be no low points or traps in the conduit that
will not drain into the manholes.

Pig. 81

Figs. 30 and 31 show two arrangements of conduit used
for distributing power from the Niagara Falls power station.*
These are made of clay ducts laid in cement and covered, as
shown, with concrete. The arrangement shown in Fig. 30
was used whenever the sewers were low enough to admit of
good drainage, because it allowed a more convenient arrange-
ment of cables in the manholes than the grouping shown in
Fig. 31. Drainage was provided by the drain tiles a, a

*L. B. Stillwell, Transactions American Institute of Electrical
Engineers, Vol. XVIII.

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38 LINE CONSTRUCTION §24

surrounded by loose gravel. These conduits are arranged
so that there is never more than one duct between any duct
and the ground, the object being to facilitate the dissipation
of heat generated in the cables.

32. Bltumlnlzed-Fiber Conduit. — ^Another kind of
conduit that has recently been introduced is made of fibrous
material treated with bituminous compound in such a way as
to make a hard, dense tube. This conduit is light, strong,
impervious to moisture, and has high insulating properties.
Joints are made by fitting the lengths together in the same
way as the pump-log conduit. Before placing a length in
position, the end is dipped in hot pitch, or similar compound,
so that when the end is pushed in, a water-tight joint is
formed. The ordinary size of this conduit is 3 inches inside
diameter and it is made in 7-foot lengths. The wall of the
tube is about f inch thick. The conduit is usually laid in
concrete, as described for the clay conduit, but owing to
the nature of the joints it is not necessary to use mandrels
if ordinary care is taken.

MANHOLES

33. Manholes form a very important part in cable sys-
tems and require careful designing to properly adapt them
to the particular conditions to be met. They are usually
placed about 400 feet apart, and, if possible, at the inter-
section of streets. They should be located with a view to
making the line of conduit between them as nearly straight
as possible. The size of the manhole will depend on the
number of ducts that are to be led to it, as well as the num- .
ber of men that will be required to work in it at one time.
Manholes 6 feet square and from 5 to 6 feet high will usually
be required for large systems, while for smaller systems, or
the outlying portions of large ones, they may be made as
small as 4 feet in length, in the direction of the conduit,
3 feet wide and 3 or 4 feet high.

Manholes may be constructed of either concrete or hard-
burned brick laid in Portland-cement mortar. The foundation

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§24 LINE CONSTRUCTION 39

should consist of a layer of concrete at least 6 inches thick.
The walls, if of brick, should be laid in cement mortar, and
should, also, be thoroughly plastered on the outside with the
same mortar. They should never be less than 8 inches
thick, and should be made double this thickness where large
manholes are constructed in busy streets. As the brickwork
is laid up, the supports for the iron brackets that hold the
cables around the sides should be built in. The roof should

^ipe.

Fio.32

be of either arched brick, concrete, or structural iron, sup-
porting some form of cast-iron manhole cover, of which
there are several types on the market.

34. Fig. 32 shows a cross-section of a ventilated man-
hole well suited for ordinary power-distribution work. It
has been found better, on the whole, to provide manholes
with ventilated covers and good sewer connections than to
close them up tight, as was formerly done. If they are
tightly sealed, gases are liable to accumulate and cause
explosions. In Fig. 32 the manhole is provided with two

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40 LINE CONSTRUCTION §24

sewer connections, so that in case the bottom one gets
clogged up, the water will be able to flow through the side
connection instead of backing up into the ducts. Both con-
nections are provided with traps to keep out the sewer gas,
and the bottom connection is equipped with a backwater
valve to keep water from backing into the manhole. A
removable cover is provided at the backwater valve, so that
any dirt that accumulates can be cleaned out.

The roof of the manhole is made by laying 3" X 3''
I beams across the top and filling between them with brick,
the whole being covered with a layer of cement. The man-
hole cover may be either round or rectangular, the round
type being preferred. Fig. 33 (a) and (d) shows two sectional
views of the style of manhole used with the conduit shown
in Figs. 30 and 31. The roof of this manhole is made
of concrete arches supported by the side wall and by two
I beams, as shown; a, a, a are the ducts of the main conduit,
and d, b the ducts of the conduit through which the branch
lines are taken. The cables pass around the side of the
manhole, and are held in place on the racks Ry R. The manhole
is provided with a sewer connection at 5, and the drains
that run alongside the conduit also attach to the sewer
connection, as shown.

35. Fig. 34 (a) shows an elliptical manhole made of con-
crete. This shape of manhole is becoming popular because it
allows the cables to be easily bent to lie against the sides of
the manhole. The rectangular comers of a square manhole
are practically waste space, because the cables cannot be forced
into these comers, or if the attempt is made to force them
in, they are almost sure to be damaged. The elliptical form
therefore utilizes the material to the best advantage. The
main features of the construction are shown by the figure, so
that little explanation is necessary. The main part a is of
concrete, molded in a suitable form, and in this case the
conduit b is of the 9-duct multiple type. The 2" X 4" tim-
bers c are built into the concrete to form a base for the
cable brackets. This manhole is comparatively small, so

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QIQ

-,»,»-

"V^-*

-3CU

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§24 LINE CONSTRUCTION 43

that the holder d for the cast-iron cover e, forms the roof.
This manhole, like nearly all those now constructed, is of the
ventilated type. In case manholes are situated above the
level of the sewer, the water that accumulates in them is
usually removed by means of a water siphon. Fig. 34 {b)
shows the cast-iron roof and cover.

36. After all work on the conduit and manholes has
been completed, the cables are drawn into the ducts. In
ord^r to do this, it is necessary to have a wire or rope pass-
ing through the duct; this is introduced by the process
called redding, which consists in pushing a number of
jointed rods into a duct from one manhole until the first rod
reaches the other manhole. The rods are joined together
by screw connections or bayonet joints, as they are pushed in.
When the chain of rods reaches between the two manholes,
a rope or wire is attached to one end arid pulled through, the
rods being disjointed one by one as they reach the second
manhole.

The introduction of the wire into the duct may often be
greatly facilitated by using, instead of the rods, a steel wire
about i inch in diameter and provided with a ball about 1 inch
in diameter at its end. This wire may be pushed through a
smooth duct without trouble for distances up to 500 feet.
If an obstruction is found during the rodding that cannot be
removed by means of the rods or by water, the distance to
the obstruction can readily be measured on the withdrawal
of the rod. The conduit should then be opened, the difficulty
removed, and the structure repaired. This difficulty, how-
ever, should never be met when proper care is taken in
laying the conduit.

37. Drawing In. — The process of drawing In the

cable is illustrated in Fig. 35. The cable reel should be
mounted on horses, so as to be free to revolve in such a
manner that the cable will unwind from its top. The end of
the rope leading through the duct should then be attached
to the cable by grips made specially for the purpose or by
binding it with iron wire for a distance of 18 inches or 2 feet

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LINE CONSTRUCTION

§24

from the end. Fig. 35 {d) shows a section of a cable grip
of iron pipe made to fit the cable snugly. It is fastened to
the cable, as shown, by common wood screws, and the piece d
to which the drawing-in rope is fastened is screwed into the
end of the iron pipe. Another form of cable grip is shown

Pig. 86

in Fig. 36. Whenever a hole is made in the end of the cable
for fastening the drawing-in rope, the end should be cut oflE
when the cable has been drawn in, the xnoisture driven out,
and the end sealed if a joint is not to be made at once. The
other end of the rope is passed over the grooved rollers,
arranged on heavy planks mounted in the distant manhole,
as shown, and is secured to a capstan or some form of
windlass, by which a slow and steady pull may be exerted.

Pio. 86

A man should be stationed in the manhole at which the
cable enters to properly guide the cable into the duct, to
prevent it from being kinked or unduly strained. It is well
to use a special funnel-shaped guide, made of wood or lead,
at the entrance of the duct, in order to further insure the
cable against injury by the corners of the duct. This guide

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§24 LINE CONSTRUCTION 45

is shown in Fig. 35 {a). It is sawed longitudinally into two
sections, as shown in the left part of Fig. 35 (a), where the
cable is to continue on through a manhole and where it
would therefore be impossible to remove the cylindrical
protector were it not sawed in two. Fig. 37 shows another
arrangement for drawing in cables. In this case the windlass
is arranged vertically in the manhole itself.

DISTRIBUTION FROM MANHOLES

38. Cables. — The construction of the eables themselves
depends on the kind of service to which they are to be put.
Two kinds of insulation are available — rubber and paper.
With good rubber insulation, a small puncture in the lead
sheath may not impair the insulation for some time, because
the rubber is, to a large extent, proof against moisture.
On the other hand, paper insulation will be damaged if the
lead sheath becomes punctured so as to admit moisture.
Paper insulation is, however, cheaper than rubber, and if
the cables are carefully installed will give excellent service.
Fig. 38 shows a paper-insulated cable designed for 6,600-
volt, three-phase transmission. The three conductors are
insulated with paper wrapping to a thickness of i inch.
These three strands are then twisted together and covered
with a wrapping of paper tV inch thick, over which the
i-inch lead covering is forced. The paper is treated with
insulating compound and the space between the strands,
shown black in the figure, is filled with jute treated with
insulating compound.

39. Underground cables have been regularly operated in
America at a pressure of 25,000 volts. These cables were
made for the St. Croix Power Company, and both paper-
insulated and rubber-insulated cables were installed, the
construction of the cables being similar to that shown
in Fig. 38. The paper insulation on each conductor is
A inch thick, and the outside paper jacket is -gV inch
thick. In the rubber cable, the insulation on each con-
ductor is A inch thick, and the jacket surrounding the

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46 LINE CONSTRUCTION ' §24

conductors is W inch thick. The sheath is of lead with 3 per
cent, of tin added.

40. Junction Boxes. — In underground electric-power
distribution, it is important to have the various parts of the
system so arranged that they can be disconnected, if neces-
sary, because faults are liable to develop, and if the various
sections can be readily disconnected, it makes the location
of the defective portion very much easier to find; also, when

Pio.87

the defective part is located, it can easily be cut out without
interfering with the operation of the remainder of the system.
Again, at a manhole or other distribution center, where a
number of distribution cables are connected to the main

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§24 LINE CONSTRUCTION 47

feeders running to the power station, it is necessary to insert
fuses, so that any branch will at once be cut off from the
main cables in case of an overload, short circuit, or other
defect giving rise to a rush of current. On low-pressure net-
works, the distribution cables are attached to the main cables,
or feeders, by means of junction boxesy which are provided
with suitable fuse terminals. Junction boxes are made in a

Fig. 38

great many different styles, but they are usually in the
form of cast-iron boxes, containing suitable fuse-contact
terminals and arranged so that they can be fastened to the
side walls or roof of the taanhole. These' boxes must of
course be water-tight.

41. Fig. 39 shows a typical junction box designed for
fastening to the side walls or roof; it is known as a four-way
box, because it accommodates four positive and four negative
branch cables; it is designed for use on low-pressure, three-
wire work. A and B are the positive and negative bars,
which are made of copper and are well insulated from
each other. These bars are connected to the cable terminals
through copper fuses /, so that in case a short circuit occurs
on a line, the fuses will blow and thus prevent damage.
The short neutral bar shown in the bottom of the box
attaches directly to the cables, because it is not usually con-
sidered necessary or even desirable to place a fuse in the
neutral. The small wires py p are pressure wires that run
back to the station and there connect to the voltmeter, so that
the voltage at the center of distribution, represented by the

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LINE CONSTRUCTION

§24

junction box, may be determined at any time. These pres-
sure wires are protected by fuses placed in the small fuse
receptacles by by b. Each pressure wire connects to one
side of a cut-out b and the other sides connect to the + , — ,

zrsi

^ — ..- — ^J

Pio. 39

and neutral bars. The cables pass into the box through
water-tight rubber gaskets, and the box is closed by a
water-tight cover.

Fig. 40 shows a recent type of junction box ma,de by
the General Electric Company. This differs considerably
from those of the ordinary type, as it is designed to be
placed in the roof of the manhole and access gained to it
from the street. In many manholes there is very little room
for placing junction boxes on the side walls without interfer-
ing with the cables, and moreover manholes are sometimes
filled with gas or water so that it is a difficult matter to get
at the boxes to replace fuses or disconnect defective cables.
Fig. 40 (a) is an exterior view of the box and {b) shows it

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§24

LINE CONSTRUCTION

located in a manhole. All cables enter through the bottom,
the lead sheath being: joined to a nozzle by means of a
wiped joint and the nozzle secured ag^ainst the box by means
of a union, as shown, thus making a joint that is gas- and
water-tight, yet easily connected or disconnected. Fig. 40 (c)
shows the arrangement of the fuses. The main cables
connect, through fuses, to the castings a, b,c and the branch
cables are connected to these through fuses d, e, etc. The box

LJU r^_U

PiO. 40

is intended for a three-wire system and i, 2, 3 are small blocks
to which the pressure wires are connected. In Fig. 40 (^),
the location of the junction box /, with reference to the
manhole opening ^, is shown. The junction box is made
water-tight by means of the inner cover k, which is screwed
down against a gasket. After the box is installed, a small
hole is made close to the inner cover and opening into the
manhole; this prevents any great accumulation of water

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50 LINE CONSTRUCTION §24

between the inner and outer covers, so that there is little
tendency for the gasket to leak. The junction box is covered
by a loose cover k similar to that used for the manhole. If
desired, the lower part of the box can be filled with oil,
similar to that used in transformers; this is advisable with
paper-insulated cables, as the oil will prevent moisture from
working its way into the insulation.

42. Service Boxes. — When the conduit system of dis-
tribution is used, and where customers have to be supplied,
small handholes are provided wherever distributing points
may be necessary. These are much smaller and shallower
than manholes and only run down as far as the conduit.
In these handholes a service box is placed. Fig. 41 shows

Pig. 41

one style of service box with its cover removed. A, B, and C
are the main cables that run straight through the box without
being cut. Z>, E are the three- wire branch-service cables, or
tubes, for supplying current to the buildings. These are
attached to the main pables by means of suitable clamps, and
after the cover is bolted in position the box is filled with
insulating compound. Fig. 42 shows another style of service
box for use on the three- wire system. In this four- way box
the main cables are fastened to terminals instead of passing
straight through. Fig. 43 shows a handhole with its service
box arranged for delivering current to overhead conductors.
The main feeders, running from manhole to manhole, are
placed in the lower tiers of conduits, and the service mains

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§24

LINE CONSTRUCTION

that run back from the manholes are nin in the upper
row, so that they will be accessible for the connection of
service boxes.

43, Joining Cables. — For low-pressure work, cables
are usually joined in the manholes by means of coupling
boxes or junction boxes. Sometimes, however, joints must
be made without the use of these boxes, in which cases the
job must be very carefully done.

First, the soldered

end of the cable is cut
off and the cable care-
fully examined for
moisture. If a little
moisture be present
and there is still more
than enough room for
the joint, it is allow-
able to cut off another
short length. If indi-
cations of moisture
are still present, heat
should be applied to the

Pio. 42

lead covering, starting from a disjtance and proceeding along
the cable to the end. Thus, the moisture is driven out at the
cut. When the use of torches is not allowed on account of gas
in the manholes, hot insulating compound, such as boiling
paraffin, may be poured over the cable. This process is
known as boiling out. To ascertain whether moisture is
present, the piece last cut off is stripped of its lead covering
and plunged into hot insulating compound. If bubbles rise,
moisture is still present.

44. Higli-Tension Cable Joint. — Fig. 44 shows a
typical lii|3cb-tension cable joint. After all moisture has
been driven out, the lead sheath is cut off for a suitable dis-
tance from each end and the cable insulation is also cut
back as indicated. A piece of lead pipe A of considerably
larger diameter than the cable and a little longer than

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52 LINE CONSTRUCTION §24

the total length of sheath stripped off is then slipped back
on the cable. A copper sleeve (d) connects the abutting
ends of the cable, and is sweated in place with solder
worked in through the slot in the top of the sleeve. The
sleeve is then covered with tape until it is brought up to a
level with the cable insulation and a paper insulating sleeve c
that has previously been slipped back over the cable insula-

PlO. 4S

tion is placed over the joint and held there by a wrapping of
string. The lead sleeve is now slipped into place and the
ends hammered down around the cable sheath as indicated,
and then soldered to the sheath with a plumber's wiped
joint. These joints should be very carefully made so that
there will be no opportunity for moisture to work into the
cable and thus cause a breakdown. Two V-shaped openings
are made in the top of the sleeve by cutting the lead and
turning it back, as shown in (c); through one of these hot
insulating compound is poured until the joint is filled. One

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§24 LINE CONSTRUCTION 53

of the openings allows the air to pass out while the compound
is poured in at the other. In joining high-tension cables, the
greatest care must be taken to have the joint perfect in every

Pig. 44

particular. A slight defect may lead to a serious breakdown
after the cable has been in use a short time.

EDISON UNDERGROUND-TUBE SYSTEM

45. The Edisou undergrrouiid-tube system differs
from the conduits previously described in that the con-
ductors are placed in iron tubes
that are buried in the ground.

The conductors are, therefore, *^

not removable. This arrange-
ment has been used extensively

for three- wire 110-220 volt dis- ^omoound

tribution in the larger cities.
The conductors themselves are

usually in the shape of round copper rods; the main tubes
are designed for use on the three-wire system and are,
therefore, provided with three rods, as shown in the section
in Fig. 45. Each rod is wound with an open spiral of
rope that serves to keep the rods separated in case the
insulating material in the tubes should become soft. After

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LINE CONSTRUCTION

§24

the rods have been provided with the rope spiral, they
are bound together by means of a wrapping of rope and
inserted in the iron pipe, the rods projecting for a short
distance at each end. The whole tube is then filled with an

(a)

insulating compound that becomes hard when cold. The
tubes are made in 20-foot lengths and are laid in the ground
about 30 inches below the surface of the pavement. They

are joined together by means of the
coupling boxes shown in Fig. 46 {a)
and {b). Fig. 46 {a) shows the
lower half of the box only, with the
main tubes entering each end. The
conductors are connected together
by means of short, flexible, copper
cables c, c, c, provided with lugs b, b,
that fit over the rods and are sol-
dered in place. A cover d similar to
the lower half e is then placed in position and the two securely
bolted together by means of flange bolts, as shown in (b).
After this has been done, melted compound is poured
through an opening in the upper casting and the joint is
complete. Fig. 47 shows two styles of connectors used
for connecting the ends of the rods; {a) is a stranded

(a)

Pio. 47

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§24

LINE CONSTRUCTION

copper cable with terminals, and {d) is a laminated cop-
per connector. Fig. 48 indicates a length of pipe with
a coupling.

46. Where branches are taken off the mains, T coupling
boxes are used, as indicated in Fig. 49, This box, also, is
filled with insulating compound that soon becomes hard and
prevents the flexible connections from coming in contact
with one another. At the centers of distribution (usually a

street intersection) jimction boxes are provided; these cor-
respond to the manholes of the conduit system. The main
supply wires, or feeders, run from the station to these junc-
tion boxes, whence the mains are run to the various districts

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56 LINE CONSTRUCTION §24

where light or power is supplied. Fig. 50 shows one of
these junction boxes. The tubes enter at the lower part of
the cast-iron box, and the mains are connected to the feeders
through fuses that bridge over between the rings shown at
the top. These fuses must be proportioned according to
the size of the conductor in the tube to which they are

Pio. 50

connected. The allowable carrying capacities of underground
tubes and cables have been made the subject of a large
number of tests by the manufacturers, who furnish tables
giving the limit to which their cables or tubes may be loaded
with safety. The junction box shown in Fig. 50 is made
water-tight by clamping down the cover by means of the

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§24

LINE CONSTRUCTION

studs by b, and the whole is then covered with a cast-iron
plate resting in the groove c and coming flush with the
street surface.

47, The underground tubes and fittings are rather
expensive, but they are comparatively cheap to install, as
all that is necessary is to dig a shallow trench and lay the
tubes in the ground. This system has the disadvantage
that if any trouble occurs it is somewhat awkward to get at

TABIiE XI

CARRYING CAPACITY OF UNDER-
GROUND TUBES

Size of Each
Conductor

Circular Mils

Maximum Current

in Each of Two

Conductors

41,000

100

80,000

200

100,000

235

120,000

260

150,000

295

200,000

350

250,000

400

300,000

450

350,000

495

400,000

540

450,000

580

500,000

620

it, as the conductors cannot be pulled out as in a conduit
system. When trouble occurs, the usual method of pro-
cedure is to dig a hole at one of the couplings and separate
the ends. By making a few breaks in this way at different
points, the section in >yhich the ground or short circuit is
present can soon be located and the defective length of tube
removed. Another and quicker method of locating ground^
will be described later.

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68 LINE CONSTRUCTION §24

48, The Edison tube system is not now used as lars:ely
as it once was for the main distributing lines or feeders.
The present practice is to carry the main conductors from
the station to the various distributing points in ducts, so
that they may be drawn out if necessary. The tube sys-
tem is, however, well adapted for the distributing mains,
and is largely used for this purpose, because it allows ser-
vice connections to be made easily and cheaply. Table XI
gives the cross-section of the rods used in the standard
tubes that are now used for distributing mains. Each tube
has three conductors of the same size, and the table shows
the allowable current when two of the conductors are loaded.
If the system is balanced, the third wire will carry but a
small current. ^_^____

TESTS

49, In testing lines or apparatus, it is frequently neces-
sary to make rough tests that will show whether or not
circuits are continuous, broken, crossed, grounded, or
properly insulated. These tests do not require accurate
measurements; they are merely made for the purpose of
determining the existence of a faulty condition.

50, Magneto Testing Set. — The most common, and
probably, all things considered, the most useful, form of
testing instrument for rough testing is that consisting of a
magneto generator and bell mounted cpmpactly in a box
provided with a strap for convenience in carrying.

TESTING I4INES FOB FAIJI-TS

51« Faults on a line may be of two kinds: the line may
be entirely broken, or it may be unbroken but in contact
with some other conductor or with the ground. The former
fault is termed a break; the latter a cross, or ground. A
break may be of such a nature as to leave the ends of the
conductor entirely insulated, or the wire may fall so as to
fprm a cross or ground. A cross or ^ound may be of such

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§24 LINE CONSTRUCTION 69

low resistance as to form a short circuit or it may possess
high resistance, thus forming what is called a leak. There
are a number of different methods used for locating faults,
and as those most suitable depend to a considerable extent
on the kind of work for which the lines are used, most of the
points relating to testing will be left until the different sub-
jects with which they are connected are considered.

52, Continuity Tests. — In testing wires for continuity,
the terminals of the magneto set should be connected to the
terminals of the wire and the generator operated. A ringing
of the bell will usually indicate that the circuit is continuous.
This is a sure test on short lines, but should be used with
caution on long lines and with cables, because it may be that
the electrostatic capacity of the line wires themselves will be
suflficient to allow enough current to flow through the ringer
to operate it, even though the line, or lines, be open at some
distant point.

53, Testing: for Crosses or Grounds. — In testing a
line for crosses or grounds, one terminal of the magneto
should be connected to the line under test, both ends of which
are insulated from the ground and from other conductors.
The other terminal of the magneto set should be connected
successively with the earth and with any other conductors
between which and the wire under test a cross is suspected.
A ringing of the bell will, under these conditions, indicate
that a cross exists between the wire under test and the
ground or the other wires, as the case may be, and the
strength with which the bell rings, and also the pull of
the generator in turning, will indicate, in some measure,
the extent of this cross.

54, Here, however, as in the case of continuity tests,
the ringing of the bell is not a sure indication that a cross
exists if the line under test is a very long one. The insula-
tion may be perfect and yet permit a sufficient current to
pass to and from the line through the bell to cause it to
ring, these currents, of course, being due to the statig

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60 LINE CONSTRUCTION §24

capacity of the line itself. In testing very long lines or
comparatively short lines of cable, the magneto set must
be used with caution and intelligence on account of the
capacity effects referred to. For short circuits in local test-
ing, however, the results may be relied on as being accurate.
Magneto testing sets are commonly wound in such
manner that the generator will ring its own bell through a
resistance of about 25,000 ohms. They may, however, be
arranged to ring only through 10,000 ohms, or where espe-
cially desired, through from 50,000 to 75,000 ohms. The
first figure mentioned — 25,000 ohms — is probably the one
best adapted for all-round testing work.

CURRENT DETECTOR GAXVANOMETER

55. In order to test for grounds, crosses, or open circuits
on long lines or on cables, without the liability to error that
is likely to arise in testing with a magneto set, a cheap form
of galvanometer for detecting currents, called a detector
gralvanometer, may be used. In testing for grounds or
crosses, the galvanometer should be connected in series with
several cells of battery and one terminal of the circuit
applied to the wire under test, it being carefully insulated
at both ends from the earth and from other wires, while the
other terminal of the galvanometer and batteries should be
connected successively to the ground and to adjoining wires.
A sudden deflection of the galvanometer needle will take
place whenever the circuit is first closed, this being due to
the rush of current into the wire that is necessary to charge
it. If the insulation is good, the needle of the gal-
vanometer will soon return to zero; but if a leak exists
from a line to the ground or the other wire with which it
is being tested, the galvanometer needle will remain per-
manently deflected.

In testing for continuity, the distant end of the line should
be grounded or connected with another wire that is known
to be good, and the galvanometer and battery applied, either
between the wire under test and the ground or the wire

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§24

LINE CONSTRUCTION

under test and the good wire. In this case, a permanent
deflection of the galvanometer needle will denote that the
wire is continuous, while if the needle returns to zero it is
an indication of a broken wire.

56. Test for Insulation Resistance. — One thing that
it is important to know about lines is the state of their insu-
lation. In order to determine this, measurements of the
insulation resistance between the line and ground must be
made, and if this resistance is found to be dangerously low,
the trouble should at once be looked up and remedied. One
of the most convenient methods for measuring insulation
resistance is by means of a good high-resistance voltmeter.
The voltmeter is much easier to handle than a reflecting

Pio. 51

galvanometer, and if the resistance of the voltmeter is
known, insulation resistance measurements may be made
with very little trouble. Suppose in Fig. 51 we wish to
measure the insulation resistance of the line A A. The
voltmeter is first connected across the lines at Fin the usual
manner and the voltage of the dynamo D obtained. Call
this reading V. After taking the reading F, the voltmeter is
connected between the line B B and the ground, as shown
at F,, and a reading F» obtained. In this case the current
passes through the insulation from / to E^ through the
ground to E^ and thence through Fi to /. It is evident
that if the insulation resistance of the line A A is very
high, very little current will flow through the voltmeter,

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LINE CONSTRUCTION

§24

and a small deflection will be the result. If the resistance r
of the voltmeter is known, then the insulation resistance of
the line will be

R = (V_=VAr (3)

Example. — The insulation resistance of an electric-light main was
tested by means of a Weston voltmeter having a resistance of 18,000
ohms. When connected across the lines, the voltmeter gave a
reading of 110 volts. When one line was connected to ground through
the voltmeter, the reading was only 4 volts. What was the insulation
resistance of the other line?

Solution.— We have by formula 8,

-, (110 - 4) 18,000 106 X 18,000
R^ 5 5

477,000 ohms. Ans.

NoTE.—The insulation resistance of lines Is usually expressed in megohms.
1 mesfohm beins: equal to 1.000.000 ohms. The resistance of the line in this case
would therefore be .477 mecrohm.

TESTS FOR GROUNDS OR CROSSES

57, Varley lioop Test. — One of the most common
methods for locating a ground or cross is by means of the
Varley loop test. In Fig. 52, C is a sensitive galvanom-
eter connected across the arms of a Wheatstone bridge in
the ordinary manner; A B and A Care the ratio arms and CD
d

_^>~~'

Fig. 52

the rheostat or balance arm of the bridge; D E v& the faulty
line, B E ^ good line, and /'is the location of the fault. The
two lines should be connected together at E and the ends of
the loop B E D,so formed, connected across the terminals of
the bridge as the unknown resistance. Call y the resistance

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§24

LINE CONSTRUCTION

of the loop from B to F and x thfs resistance from D to F.
With the battery connected between A and D, as in the
ordinary method of using the Wheatstone bridge, balance
the bridge. This will give, by working out the unknown
resistance in the usual manner, a resistance R equal to the
sum of the resistances of the two wires forming the loop; that
iSyR = y + X. Or, the resistance R of the whole loop may be
calculated, if the length and size of the line wire are known.

Pio.58

Now disconnect the battery from D and connect it to the
ground, as showti in Fig. 53. Then balance the bridge again,
and the resistance x may be obtained by means of the follow-
ing formula:

x = ^R-^P (4)

in which my n, and/ are the values of the resistances in the
arms A B^ A C, and C D. After obtaining the resistance x
from D to the fault F along the line D Ehy means of for-
mula 4, the distance (in feet or miles) from the testing endZ?
to the fault F may be obtained by dividing this resistance x
by the resistance of a unit length (a foot or a mile, as the
case may be) of the line wire D E. The result obtained by
this test is independent of the resistance at the fault
between the line and the ground.

ExAMPLB. — A gfTound occurred on a conductor of a cable 10,000 feet
long composed of three No. 10 wires. One good wire was used to

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LINE CONSTRUCTION

§24

complete the loop. On testing with one end of the battery g^rounded as
in Pig. 53, the bridge was balanced with the following resistances:
wf = 10 ohms, « = 1 ,000 ohms, P = 1 ,642 ohms. Where was the ground,
the resistance per 1,000 feet of the conductor being .0972 ohm?

Solution. — The length of the loop formed by joining the two wires
of the cable at the distant end will be 20,000 ft.; hence, i? = 20 X .9972
1,000X19.944-10X1,642

= 19.944, and x =

1,000-1-10

distance of the fault from the testing station must be
3.4891

= 3.4891. Hence, the

.9972

X 1,000 = 3,498.9 ft. Ans.

^ ^

Pio. M

58* liocatln^ a Partial Ground Wlthoat an Avail-
able Good Wire. — The following method for locating^ a
partial ground or leak is rather tinreliable in practice,
because the resistance of the partial ground may change

Fro. 65

between the two measurements, and so give a more or less
incorrect result. However, it is about the only way where
there is no available good wire and where the tests must be
made from one end only. The normal resistance of the

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§24

LINE CONSTRUCTION

line must be known from some previous measurement,
unless it can be calculated from the length and size of the
wire. Let this resistance be a; then measure the resistance
of the line B B\ with the distant end B^ gfrounded as shown
in Fig. 54, and call this c. Also measure the resistance
with the distant end open, as in Fig. 55, and call this ^ ohms.
Then the resistance x to the partial ground from the test-
ing station is given by the following formula:

x^c- ^i{b''c){a--c) (6)

Pio. 56

By dividing x by the resistance per unit length of the
wire, known from some previous measurements or by a cal-
culation from its size, length, and a table of resistances for
the kind of wire under consideration, the distance to the
groimded point may be obtained.

' Pio. 57 ^

69. To Locate a Cross by the Varley Loop Method.

First insulate the distant ends of the two crossed wires.
Then connect as shown in Fig. 56 and measure the resist-
ance from D to B through the cross F. Let the resistance
of the cross be z ohms and the resistance found by balancing
the bridge be ^ ohms. Then,

J^ = x + y-\-:s (1)

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66 LINE CONSTRUCTION §24

Now ground either wire, say D E, anjrwhere beyond the
cross, and connect as shown in Fig. 57. When the bridge
is again balanced, we have

«? = ^ (2)

From equations (1) and (2), x = VlEjuULP^

m + n

This is the same as formula 4. By dividing x by the

resistance of the wire D£ per unit length, we have the

distance from D to the fault along the wire DB.

LOCATING GROUNDS AND CROSSES ON CONDUCTORS OP
LOW RESISTANCE

60. The above tests, in which the location of a groimd
or cross is determined by means of resistance measure-
ments, are capable of giving the location quite closely,
provided the wire is fairly small, say less than No. 8 or
10 B. & S. When the wire is large, as it nearly always
is in connection with power-transmission systems, bridge
methods do not give the location close enough, because
it is evident that a small resistance corresponds to a long
length of conductor. The location of faults on these large
conductors is of special importance in connection with
underground distributing systems, and the bridge methods
cannot usually be applied on account of the low resistance
of the conductors. When a cross occurs between the con-
ductors of an underground cable, it nearly always results in
a groimd also, because the consequent short circuit fuses
the cable, thus making connection between the core and the
sheath. One way of locating faults on underground cables
is by the cut-and-try process already mentioned. A manhole
is opened at a point near the middle of the line, and the
cable is cut. Each half is then tested and the half on which
the fault exists is then cut out at its middle point, and so
on until the fault is located between two manholes. This
method is slow and expensive, especially where high-tension
cables are used, because the making of joints in such cables
is a slow and costly operation.

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§24 LINE CONSTRUCTION 67

61. Another method of locating: faults is to run a heavy
current through the cable so as to bum the insulation at the
fault, and thus fill the duct and manhole with smoke. On
opening the manholes the presence of the smoke indicates
the location of the fault. This method, while more rapid
and less expensive than the cut-and-try method, has the
disadvantages that the burning of a cable, especially if near
a manhole, is liable to injure other cables, and also that the
burning is liable to ignite accumulated gases and thereby
cause a subway explosion.

62. Fig. 58 shows, diagrammatically, a method recom-
mended by Mr. Henry G. Stott,* which is particularly useful
for locating faults on underground cables of large size. A A

Pio.68

is the cable nmning through a series of manholes Ex^ ^„ etc.
A ground has developed say at G', and this ground has to
be located. C is a small direct-current dynamo; an arc light
machine answers very well, because it maintains a fairly
constant current, irrespective of the resistance of the circuit.
B is s. current reverser, which is revolved by means of a
small motor. Brushes /, g, which press on the rings ^, <z,
are connected to the terminals of C, and the contact arcs c, d
are connected to the conductor and ground by means of
brushes.^, k. The rings a and b are connected to arcs c and d,
so that as the contacts revolve, the current flowing through
the cable to the fault G^ and back to G is periodically reversed.

*Transactions American Institute of Electrical Engineers, Vol.

xvni.

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LINE CONSTRUCTION

§24

The speed of the mo-
tor is such that the
current is reversed
once in about every
10 seconds. The fault
is located by first
opening a manhole
about the middle of
the line, say at -£"„
and laying a compass
D on the cable. The
direct current, which
need not be greater
than 8 or 10 amperes,
will cause the needle
to swing first to one
side and then to the
other every 10 sec-

. onds. If the needle
£ swings in this way at
^„ it shows that the
fault is beyond E^\
hence, by this test,
one-half of the cable is
eliminated. The man-
hole is then closed
and another test made
at say E^, At E^ no
reversals of the com-
pass will be obtained,
because the current
does not flow in the
cable beyond the
fault. The fault- is
therefore located be-
tween E^ and E^\ by
opening a few inter-
mediate manholes the

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§24 LINE CONSTRUCTION 69

defective part is soon located between E^ and E^y and this
section of cable can be removed and the fault remedied. It
will be noticed that, with this method, the cable is not cut
and the time required to make the test in each manhole is
very short, so that the trouble is quickly located, and there
are no joints to be made afterwards save those actually
needed to replace the defective part of the cable.

In case the cable system carries alternating current and
has no permanent ground attached to it, this device may be
used for locating a fault even while the alternating current
is on the system. The testing device is simply connected to
the feeder network as shown, but in series between it and
the network is placed a reactance coil, for example, the
primary of a transformer, the circuit being opened at e and
the coil connected in series as shown at /. This avoids
damage to the dynamo C by preventing a rush of current
from the alternating-current generators in case another
ground should occur on the other side of the system while
the test was being made, thus producing a short circuit.
Before applying the test it is a good plan to break down the
insulation resistance of the fault by applying a high potential,
between the conductor and ground, for a few seconds.

Fig. 59 shows the style of reverser used in applying this
test. An induction motor M drives the shaft s by means of
a worm-gear. The two-part commutator revolves in oil so
as to give a quick reversal of the current.

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SWITCHBOARDS AND SWITCHBOARD
APPLIANCES

SWITCHBOARD APPLIANCES

SWITCHES

1. Introduction. — The methods available for the
transmission of. electrical energy have been described in a
general way, and it will now be necessary to examine more
closely the various devices that are used for the control of
the output of the generating plant. In order that a trans-
mission system shall be under control, and also that the
amount of the output, the condition of- the lines, etc. shall be
known, it is necessary to have various controlling and pro-
tective devices in the station. These are usually grouped
together at one central point on the switchboard^ and consist
of switches, fuses, circuit-breakers, ammeters, voltmeters,
ground detectors, lightning arresters, power factor indicators,
wattmeters, and other auxiliary devices.

2. Probably the most important appliances on the switch-
board are the s^wltches, which are used for connecting or
disconnecting circuits or dynamos from the rest of the sys-
tem. Switches must be carefully selected with a view to
the work that they have to perform. They must have ample
carrying capacity and be capable of breaking the full-load
current of the dynamo or circuit, without destructive burning
or arcing. The style of switch used for any installation will
depend on the voltage and current to be handled. For

For notice of copyright, see page immediately following the title page.

135

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SWITCHBOARDS AND

§25

convenience, we will consider switches as divided into two
classes: low-tension^ for handling pressures up to about
1,000 volts, and high-tension y for pressures above this amount.

LOW-TENSION SWITCHES

3. For pressures up to 1,000 volts, plain knife switches
are generally used, though this style of switch with a broad
separation of the blades and contacts has been used on

pressures as high as 2,500 volts.
For work of the latter class, how-
ever, it is preferable to use a
switch of the quick-break variety,
and even for pressures of 500
volts, quick-break knife switches
are commonly used. Fig. 1 shows

Pio. 1 Pio. 2

a typical two-pole knife switch designed for front connec-
tions and provided with fuses. Fig. 2 shows a similar switch
without fuses and intended for mounting on a switchboard.
When the switch is opened, connection is broken between
the two clips at each side, thus opening both sides of the
circuit. Knife switches should be substantially constructed
and should have a contact surface at the clips of at least

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§25

SWITCHBOARD APPLIANCES

into tr-
to ak

SWItdn

abrojL
used o:
)0 voiti.
5S, how-
) use a
variety,
of •^-'
vitchef
showi

1 square inch for every 50 to 100 amperes, the allowable cvtr-
rent density being greater in small switches than in large
ones. Bolted contacts will carry 200 amperes per square
inch, and laminated contacts, such as are described later
on ip connection with circuit-breakers, will carry from 300 to

TABIiE I
CURRENT DENSITIES FOR COPPER STUDS

Diameter of

Stud

Inches

Current Density

Amperes
per Square Inch

Diameter of
Stud
Inches «

Current Density

Amperes
per Square Inch

i
i

1,200
1,150
1,100
1,000

2
3

950
850
800
700

500 amperes per square inch. For copper studs the current
densities, shown in Table I, should not be exceeded if the
temperature rise is to be limited to about 20° C.

For the same temperature rise the current density must be
smaller in large studs than in small ones, because in the
large studs the heat is not so readily radiated.

ffi

Fig. 3

ES^

----di

4. The blades should be made of good conducting mate-
rial, preferably of drawn copper, and the clips should be stiff
enough to give a good, firm contact. For pure copper, the
blades should have a cross-sectional area of about 1 square inch

45—22

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SWITCHBOARDS AND

§26

per 1,000 amperes. Fig. 3, together with Table II, shows the
dimensions, in inches, of General Electric knife switches.

Knife switches should always be mounted with the handles
up; this is in accordance with a rule of the Fire Underwriters,

lBI

•

DIMENSIONS OF KNIFE

SWITCHES

Capacity

Dimensions Common to All

Sinrle-Pole

Donble-
Pole

Triple-
Pole

Pour
Pole

Amp.

Volts

125

i«

6ft

4ft

125

i«

6ft

4»

100

125

250

6ft

sft

Sft

250

6ft

100

250

2«

7ft

200

125-250

6tt

300

125-250

500

125-250

iii

I3i

800

125-250

12*

I3f

I3i

I4i

1,200

125-250

=.*

I2t

i3i

I3i

1,500

125-250

lol

•3

13I

13*

which requires switches to be so placed that when open
they will not tend to fall closed of their own accord.

5. Fig. 4 shows a style of quick-break switch that has
proved very successful and is suitable for pressures as high
as 2,000 to 2,500 volts if the current is not large. It has

been very widely
used on direct-
current railway
switchboards. The
switch blade, of
drawn copper, is-
made in halves
Ay By which are connected by two springs r, one on each side
of the blade. When the handle is pulled out, the half A
leaves the clip E and thus stretches the springs. When the
bottom blade flies out, it leaves clip E very quickly, thus
drawing out the arc and breaking it almost instantaneously.

cfisaa:^^

P10.4

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§25 SWITCHBOARD APPLIANCES 5

HIGH-TENSION SWITCHES

6. In long-distance transmission plants using alternating:
current, the pressures are very high, and in some cases also
the volume of current is large. A switch to interrupt a
heavy current at high pressure has to be carefully designed,
and a great many types have been brought out. These may
be divided into three general classes:* (1) Those in which
the arc is interrupted m the open air; (2) those m which the
arc is interrupted in a confined space; (3) those in which
the arc is broken under oil.

These switches may be arranged for hand operation or
they may be designed to operate automatically in case the
current exceeds the allowable limit. If used in the latter
way, they are generally called circuit-breakers to distinguish
them from the non-automatic type. In many cases it is
necessary to have high-tension switches arranged so that
they may be operated from a distant point, because it is not
practicable or even desirable to have high-pressure switches
of large capacity mounted on or near the operating board.

SWITCHES BREAKING ARC IN OPEN AIR

7. In this type of switch the arc is simply pulled out
until it is broken. Fig. 5 shows a modification of the switch
shown in Fig. 4. This switch will handle a moderate cur-
rent at pressures up to 5,000 or 6,000 volts, but where the
volume of current is large, it is better to use a switch
belonging to class (3).

The switch (Fig. 5) is constructed .so as to give a long,
quick break, and is mounted on grooved insulators i, 2, 5, 4,
This insulating material passes through the panel, so that in
no place does the metal switch stud come in contact with the
marble. This is a necessary precaution in cases where very
high pressures are handled, because the marble cannot be
depended on to give good enough insulation. Blade A has

*Classification given by E. W. Rice, Jr. Transactions American
Institute Electrical Engineers, Vol. XVllI.

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SWITCHBOARDS AND

§25

a hole in the end instead of a handle, and the switch is
pulled open by means of a hook in the end of a handle about
3 feet long, thus allowing the attendant to stand back
some distance and avoid the danger of being burned by the
arc. To avoid arcing from one switch to the next, marble
barriers C are mounted at right angles to the main part of
the board.

For handling very high pressures, such as 20,000 volts and
upwards, air-break switches have been used to quite a large
extent. In these switches, the movable contact is generally

mounted on one end of a long arm,
so that when the arm is thrown
out, a break of several feet is made
in the circuit.

8. Stanley Plug Switch.

Fig. 6 shows a type of air-break
switch made by the Stanley Elec-
tric Manufacturing Company, and
used on pressures as high as
30,000 volts, at which pressure it
is capable of handling a current
of 25 amperes. A long wooden
handle a is provided with a ter-
minal b on its upper end, and this
terminal is connected to a plug c
by means of a flexible cable d. When plug c is inserted,
it makes contact with a terminal sunk well below the
surface of the marble, where it cannot be touched acci-
dentally. Also, it is locked in position, so that the circuit
cannot be accidentally opened at this point. The terminals
e and / are mounted on ribbed porcelain insulators, and are
made in the form of tapered points, as shown, so that the
tip b may be slid over them. Hard-rubber guides arranged
below the porcelain insulators engage with the projection
cast on by so that the handle a must be pulled straight
down for a short distance when the switch is being
opened, thus preventing terminals <?, / from being bent.

Pio. 5

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§25 SWITCHBOARD APPLIANCES 7

When the handle has been pushed up into place, it is

held by clamps gy h. The switch shown in Fig. 6 is-

of the double-throw type, i. e.,

terminal c can be connected to

either e or /; sl marble barrier k

is placed between the terminals

to prevent arcing across. When

the switch is opened, the handle

is pulled down until the contact

is separated from the taper plug,

and it is then swung back over the

operator's shoulder and moved

away from the board until the

arc is ruptured. The tapered

terminals and the terminal on

the handle are provided with

zinc tips, as it has been found

that the arcing does not roughen

up the zinc to the same extent

as copper. One advantage of

this type of switch is that the

live terminals are at the top of

the board out of reach of the

operator. By unlocking plug r,

the handle with its cable may fio- «

be removed entirely if it is desired to clear the board.

SWITCHES BREAKING THE ARC IN A CONFINED SPACB

9. Westlnghouse Plunger Switch. — Fig. 7 shows a
Westinghouse switch where the arc is broken in a confined
space. The terminals are mounted at each end of a porcelain
cylinder. A copper rod or plunger passing through these
contacts or bushings completes the circuit, and when the
plunger is withdrawn, the arc is formed in the confined space
between the bushings. A small outlet is provided in the
side of the tube, and when the arc is formed, the blast caused
by the sudden expansion of the air in the confined space,
together with the cooling action of the porcelain wal's,

Digitized by VjOOQIC

8 SWITCHBOARDS AND §25

extinguishes the arc. If the pressure to be handled is very-
high, a number of these
cylinders are connected in
series, thus producing a
long break. The cylinders
I, 2, 5, etc. and plungers
1\ 2^ 3' are mounted on
the back of the board and
are operated by a lever on
the front. In the figure the
switch is shown thrown
out, but when the plunger
is in, bushings a and b,
c and d are connected to-
gether, and the path of
the current is a-b-e-d-c to
^® -^ line. When the plunger is

withdrawn, the arc is broken between a and ^, c and d.

10. Stanley Slide Switch and Circnit^Breaker.

Pio. 8

Fig. 8 shows a Stanley slide switch provided with an

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§25

SWITCHBOARD APPLIANCES

automatic attachment that will open the switch whenever the
current exceeds the amount for which the circuit-breaking
device is adjusted. The attachment consists of a solenoid a
through which the main current flows. When the current
exceeds the allowable amount, the solenoid releases a catch
and a spring throws the switch out. If it is not desired to
use the switch as a circuit-breaker, the automatic device can
be cut out. The switch terminals are mounted in the insu-

cz=:&I>^

(h)

Fio.9

lating blocks *, V, of which there are two for each pole; in
this case there are six terminals, the switch being three-pole.
For each pole there is a cross-piece ^ provided with blades d, d'
that make contact with the terminals when they are forced in
by swinging the handle d up. The motion of d is transmitted
to the cross-pieces c by means of a rack and pinion, and when
the switch is opened the blades are withdrawn from the

Digitized by VjOOQIC

10 SWITCHBOARDS AND §25

clips; as soon as they leave the insulating pieces, a shutter
arrangement closes the opening, thus preventing the arc from
following the blades. Switches of this type are made in a
number of different sizes and are capable of handling as high
as 60 amperes at 3,300 volts. The present practice, however,
is to use oil switches for most high-pressure work.

11. Stanley Stab Switch. — Fig. 9 shows a simple form
of high-tension switch that is capable of handling a current
of 10 amperes at pressures as high as 7,000 volts. When the
rod a is inserted, contact is made between the bushings b, c
mounted in a thick fiber insulating tube. When a is with-
drawn, the marble ball d drops from the cavity e in which it
is held by the rod, and takes the position shown, thus
effectually smothering the arc. The vent / provides an exit
for the heated air. Switches of this type are particularly
adapted for high-pressure, series-arc lighting circuits or
series-incandescent lighting circuits.

SWITCHES BREAKING ARC UNDER OIL

12, It has been found. that circuits carrying large cur-
rents at high pressure can be successfully broken by sepa-
rating the terminals under oil, and oil-break switches have
come much into use within the last few years. Circuits in
which there is more or less inductance, producing a lagging
current, require more effective switching devices than those
in which there is no inductance, because the induced E. M. F.
always tends to prolong the arcing when the switch is opened.
Oil switches have proved very efficient on circuits of this
kind. As soon as the switch terminals are separated under
oil, the oil fills the gap and arcing is effectually suppressed
with a comparatively short separation of the terminals. It
was at first thought that the very sudden break caused by a
switch of this kind might give rise to severe strains on the
insulation of the system, but this has not proved to be the
case, and oil switches are now very largely used, both in
central stations and also in connection with motors or othei
apparatus using alternating current. There are many different

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§25 SWITCHBOARD APPLIANCES 11

reliable makes of oil switches, but for purposes of illustration
we will select a few examples of the General Electric type.

13. General Electric Oil Switches. — Fig. 10 (a) and
(d) shows a switch designed for mounting on the front of
the switchboard or for
individual use with
motors or other appa-
ratus. The same style
of switch is made for
mounting behind the
switchboard with the
operating handle on
the front of the board;
(d) shows the switch
with the oil tank re-
moved. In this case
a triple-pole, single-
throw switch is illus-
trated, though the
same type is made in
single-pole, double-
pole, and four-pole, and
for either single-throw '
or double-throw. The
terminals «, a, a are
mounted in the porce-
lain insulators ^, ^, b.
The contacts c are
hinged as shown, and
are connected together
by a wooden cross-
piece e connected to
the operating handle.
The other contacts d (^)

make a firm wiping fio. lo

contact with c when the switch is closed. The wires leading
to and from the switch are attached to the terminals a, a, a

Digitized by VjOOQIC

12 SWITCHBOARDS AND §25

so that they do not pass through the oil tank, and there
is, therefore, no chance for oil leakage if the tank is not filled
too full. This type of switch is recommended for use with
all inductive appliances, such as induction motors, that
operate at 250 volts or higher. It is hot intended for circuits
operating at pressures higher than 3,500 volts or in cases
where the load exceeds 850 to 1,200 kilowatts, three-phase,
under emergency conditions; i. e., under a short circuit or
very heavy overload.

14, Fig. 11 (a), (d), and (c) shows another Greneral
Electric switch of larger capacity. This is made single-,
double-, triple-, and four-pole, and for single-throw only. The
load that it can rupture under emergency conditions must
not exceed 3,500 kilowatts, and the pressure 6,600 volts.
For potentials exceeding 5,000 volts, the switch is not
mounted on the back of the switchboard, as shown in Fig. 11,
but is placed in a fireproof compartment entirely detached
from the board. The operating handle on the board is con-
nected with the switch by means of a series of levers. By
this arrangement, the danger of fire at the switchboard is
minimized and the operating devices can be entirely separated
from the high-tension parts of the switch. Fig. 12 shows
the general arrangement referred to, though, of course, the
actual arrangement of the levers would depend on the
relative location of the operating board and switch. These
switches are arranged for simple hand control, or they can be
provided with an attachment that will open them automat-
ically in case the load becomes excessive, thus combining the
feature of a switch with that of an automatic circuit-breaker.
Fig. 11 (a) shows the operating handle provided with the
automatic attachment; (d) shows the arrangement for hand
control; (c) shows the construction of the switch proper with
the oil tank removed. The terminals are held in the porce-
lain insulators d, b, b, which are ribbed in order to inter-
pose a large leakage surface between the terminals and the
framework of the switch. When the operating handle is
pushed in, the metal cross-pieces r, r, c are raised by th6

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§25 SWITCHBOARD APPLIANCES 13

i^/

Fio. 11

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14 SWITCHBOARDS AND §25

system of levers and brought into contact with the fingers
dy d, dy thus completing the circuit. Each cross-piece c is
attached to a wooden rod e, and these rods are attached to a
common crosshead that is moved up or down by the levers
^controlled by the operating handle. When the oil tank is in

place, the contacts c and d
are completely submerged
in oil.

15. The automatic trip-
ping mechanism used when
the switch is mounted on the
board is shown in {a). It
consists of two solenoids /,/,
which, when the current be-
comes excessive, draw up
their cores, which strike the
lever ^, g. This releases
the link h that connects the
operating handle with the
switch and allows the switch
terminals to separate. The
link k slides out through
the operating handle, but
the handle itself remains in.
The projecting link, there-
^®*^ fore, acts as an indicator

and shows that the switch has opened automatically. When
the switch is opened by hand, the button k on top of the
operating handle must first be pressed down.

16. Fig. 13 shows the connections for the tripping coils
when the tripping mechanism is placed at the switch as in
Fig. 11. The windings of the coils, Fig. 13, are connected
to the secondaries of two current transformers, the primaries
of which are in series with the mains, as shown. If the
current in the mains becomes excessive, the current in the
secondaries and tripping coils increases in like proportion,
and if the current exceeds the value for which the armatures

Digitized by VjOOQIC

§25

SWITCHBOARD APPLIANCES

of the coils are adjusted, the switch is opened by the opera-
tion of either one or both of the coils.

When the switch is not mounted on the board, the tripping
coil is operated through an overload relay or auxiliary pair
of magnets, as shown in Fig. 14. In Fig. 12, the tripping
coil is located at a, and consists of a single coil, the arma-
ture of which moves the light wooden rod b and allows the
switch to open promptly whenever there is an overload.
In Fig. 14, a is the tripping coil and b, c the coils of the over-
load relay situated on the switchboard or at any other con-
venient point. Under normal conditions the contacts </, e of

ToLoa^

Trippina Cotis
Pio.18

the relay short-circuit the tripping coil, but in case the current
becomes excessive, either one or both of the coils draw up
their cores and raise contacts d, e, thus making the current
from the series-transformers take the path through the trip-
ping coil a and opening the switch.

17. Oil Switch of liargre Capacity. — Fig. 15 shows
two views of a General Electric oil switch of large capacity
for use in central stations handling large alternating currents
at high pressure. The switch is arranged for control from a
distant point, the movements being effected by means of an

Digitized by VjOOQIC

SWITCHBOARDS AND

§25

electric motor. These switches have also been built foi
operation by compressed air, and the Westinghouse Com-
pany make a somewhat similar switch operated by solenoids.
The casing: of the switch shown in Fig;. 15 is made of brick,
and is provided with a removable iron door. The casing
is divided into three compartments, one for each phase,
and since they are separated by brick partitions, a bum-
out, if it should occur, cannot spread to other parts.
These switches are designed with a view to using the
smallest possible amount of oil, because where there are a
large number placed in a plant, the presence of a large
quantity of oil in the switches would introduce a serious

TbLoeul

Fig. 14

fire risk. In each compartment is a pair of brass cylinders
a, a with a contact sleeve at the bottom of each. These cans
or cylinders are lined with insulating material, are filled with
oil, and are provided w^th porcelain insulating sleeves b at
the top through which slide copper rods c. The two rods
are connected together by the cross-piece d^ so that when the
rods are pushed down into the contact sleeves in the bottom
of the cylinders, the two cylinders are electrically connected,
the current passing from one cylinder to the other by way of
rods r, c and cross-piece d. The cross-pieces d are attached
to a crosshead e by means of wooden rods /, and the motion
of the crosshead is controlled by means of the motor g.

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§25

SWITCHBOARD APPLIANCES

The motor is thrown into g:ear with a worm that operates a
worm-wheel in the casing h, whenever the solenoid k is
excited from the switchboard. On the worm-gear shaft is
a crank / which together with a link m forms a togglejoint.
When the switch is out, as shown in the figure, spring n is
compressed and the switch tends to close, but is prevented

Pig. 15

from doing so because the toggle / w is on center. As soon
as the motor is started from the switchboard, the crank / is
moved off center and the crosshead e is at once forced down.
The crank / is driven from the worm-gear by means of a
ratchet, so that as soon as the toggle is moved off center,

Digitized by VjOOQIC

18 SWITCHBOARDS AND §25

the crank is carried around through nearly a half revolution
independently of the movement of the motor. As soon as
the crank stops, the ratchet at once takes hold and the crank
is turned through the remainder of the half revolution until
the toggle is again on center. The switch is now completely
closed, and the motor is stopped. automatically by means of
a rotating switch moved by the worm-gear shaft. When the
switch is closed, spring o is compressed and springs p are
stretched. The switch is opened by starting the motor from
the switchboard, as before, thus throwing the toggle oflE
center again and allowing the springs to throw up the cross-
head. In the opening operation, the springs p assist spring o,
so that the opening is quicker than the closing, the time

Fio. 16

required being about 1 second. For switches that have to
handle large currents, the rods c, c are provided with auxiliary
bell-shaped contacts ^, q, which, when moved down to the
dotted position, make contact with the upper part of the
cylinders, thus relieving the rods of the current. When
the switch moves up, these contacts leave the cylinder
before the contact is broken inside the cylinder, so that no
arcing takes place at the auxiliary contacts. The cylinders are
mounted on ribbed porcelain insulators r, r, and are arranged
so that they can be easily removed from these supports. The
switch shown in Fig. 15 has a range of movement of 17 inches
and is capable of handling 300 to 800 amperes at 12,000 volts.

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§25 SWITCHBOARD APPLIANCES 19.

18. Stanley Oil Switches. — Figs. 16 and 17 show two
types of Stanley oil switch. The switch shown in Fig. 16
is of the double-pole, double-throw type with the oil tanks
mounted side by side. Fig. 17 shows a three-pole, single-
throw switch with the tanks mounted one behind the other,
so that the switch can be mounted on a narrow panel. The
oil tanks a, b, c are of cast iron with an enamel lining, and
are mounted under the marble slab d to which the fixed switch
terminals e are attached. The slab d is supported by iron
castings, and the switch arms / are operated by means of the
levers, as indicated, thus throwing the blades g into or out
of contact with the fixed clips. The terminals / are protected
by wooden boxes, and the operating handle k is thoroughly

Fig. 17

insulated from the working parts of the switch by the wooden
arm /. The tanks are arranged so that they can be easily
refilled. There are two breaks in each leg of the circuit; in
Fig. 17, for example, there are two fixed clips e in each tank,
and the two corresponding blades g are connected together.

BUS-BARS
19. Bus-bars should have a cross-section of at least
1 square inch per 1,000 amperes and should be arranged so
that the heat generated in them can be readily radiated.
They should be substantially mounted and carefully insu-
lated, particularly in cases where a high pressure is used.
The bars are usually of flat rectangular cross-section; and if

dj — 23

Digitized by VjOOQIC

SWITCHBOARDS AND

§25

large current-carrying capacity is required, a number of thin
bars are built up with air spaces between to allow ventila-
tion. Thus, a bar made up of four bars i inch thick with a
i-inch air space between each bar would be much better than
a solid bar 1 inch thick. Heavy solid bars should not on
any account be used with alternating current. Where bars
are made up of a number of thin bars with air spaces
between, joints are readily made by interleaving the bars
and bolting through, thus giving a large contact area.
Round bars and copper tubes are occasionally used for
bus-bars but they are not as desirable as flat bars except.

Bus Bar O

^ -S//7P

^S/r/p

BacA of Board

Bl
S

Bus A
Bar ^

Pio. 18

perhaps, for high-tension boards, where the current to be
handled is small and where it is desirable to have the bars
covered with insulating material.

Fig. 18 shows a simple method of mounting bus-bars for
small low-pressure switchboards. Fig. 19 shows a method
that has been largely used on 500-volt railway switchboards,

20. Carrying: Capacity of Bus-Bars. — Bus-bars should
be of liberal cross-section, otherwise the loss in them may
be considerable. For aluminum bars, a density of from 500
to 600 amperes per square inch is allowable. Cast copper
is much inferior to rolled or drawn copper as a conductor.

Digitized by VjOOQIC

§25

SWITCHBOARD APPLIANCES

and the density in cast bars, studs, or fittings should not
exceed 500 amperes per square inch. Brass can carry from
100 to 350 amperes per square inch, depending on the amount
of copper in its make-up.

21. Mounting for Hi^li-Tension Bus-Bars. — When
bus-bars have to handle a large current at high pressure, it is
very important that they be mounted so that there is
practically no possibility of a short circuit taking place
between them. A short circuit on such bars might cause a
great deal of damage, particularly if a number of machines
happened to be feeding into the bars at the time. It has

fOii

Bustior

Connection
toSwitcH

PlO. 19

become customary, therefore, in large stations supplying cur-
rent at high pressure, to mount the bars on fireproof supports
and separate them by fireproof partitions so that each bar
shall be in a compartment by itself. Fig. 20 shows the
method of mounting 6,600-volt bus-bars in a large station in
New York city. The bus-bar a is made up of four rolled
copper bars 3 inches wide by i inch thick, and is bolted to a
stud b that is covered with an insulating tube c. The bar,
with its connecting stud, is supported on a firebrick slab d,
this slab being built into the brickwork e /. Thorough
insulation is provided by the grooved porcelain insulators^,^.

Digitized by VjOOQIC

SWITCHBOARDS AND

§25

and connections are made to the bar by means of the cable
terminals A, h and plate k. Firebrick or soapstone slabs

Pig. 20

projecting at right angles to the wall e f are used as
barriers between adjacent bars.

VOIiTMETER CONNECTIONS

22. It is customary, on switchboards, to make one volt-
meter answer for several machines or circuits by providing

Fig. 21

suitable voltmeter plugs or a voltmeter switch by means of
which the instrument can be connected to the circuit or

Digitized by VjOOQIC

§25

SWITCHBOARD APPLIANCES

machine on which a voltage reading is desired. Figs. 21
and 22 show a common plugging arrangement. A pair of
voltmeter bus-wires a, b are con-
nected to the voltmeter V, Fig. 21,
and taps connect from «, b to the
plug receptacles i, 1'. The dif-
ferent dynamos are connected
to 2,2' and when a voltmeter read-
ing is desired on, say, machine A^ ^i®- 22
a plug. Fig. 22, is inserted into the receptacle, thus con-
necting i,2 and i',2'.

23. Pressure Wires. — In many cases, particularly
on systems supplying current for lighting pm-poses, it is
necessary to know the pressure at the point where the
current is utilized rather than at the station. In some
cases, especially on low-pressure, three-wire systems, pressure
wires a, b are run back to the station, as shown in Fig. 23.

f)v3^ure kf7rej.

reeder^.

Cettter

Pio.28

The current required to operate the voltmeter is so small
that there is practically no drop in the pressure wires; they
can, therefore, be of small cross-section (usually No. 8 or
No. 10 when strung on poles); insulated iron wire is some-
times used for the purpose.

24, Compensatingr Voltmeter. — In order to avoid the
use of pressure wires, compensating voltmeters, ox compensators,
are sometimes used with alternating-current circuits. The
compensator is a device used in connection with the
voltmeter to decrease the voltmeter reading as the load

Digitized by VjOOQIC

SWITCHBOARDS AND

§25

increases, by an amount proportional to the drop in the line.
The attendant then increases the field excitation of the
alternator and brings the pressure up to such an amount
that the voltage at the distributing point is correct.

Fig. 24 shows the connections for one of the earlier types
of Westinghouse compensating voltmeter. It consists of a
series-transformer with both primary and secondary coils
wound in sections. The primary is in series with the main
circuit, and the secondary cormects to a small auxiliary coil c
on the voltmeter in such a manner that the current in c opposes
the action of the current in the regular voltmeter coil d that

Pio. 24

is fed from the small potential transformer 71 When the
voltage at the distributing end of the line is at its correct
value, the hand of the voltmeter indicates the standard
voltage. When the load increases, the current through the
primary of the compensator also increases; this, in turn,
increases the current in the secondary and the auxiliary coil.
The hand on the voltmeter, therefore, goes back, and the
pressure must be raised to bring it back to the standard point.
By plugging in at different points on the primary and by set-
ting at different points on the secondary, the compensator
may be adjusted for operation on almost any of the circuits

Digitized by VjOOQIC

§25 SWITCHBOARD APPLIANCES 25

ordinarily met. After it is once set to suit the particular
line on which it is to work, it requires no^ further attention.

25. The Merslion Compensator. — The compensator
just described answers very well for lines that have little
self-induction and that supply a non-inductive load. Where,
however, the load is inductive, as, for example, a load of
motors or of motors and lamps, the reactance of the line may
have a very great influence on the drop in voltage, and the
compensator must compensate not only for the ohmic drop
in the line, but also for the drop due to the reactance. The
Merslioii compensator, brought out by the Westinghouse
Company, is designed to accomplish this.

26. The principle of this compensator is briefly as fol-
lows: The E. M. F. supplied at the end of the line is always
equal to the resultant difference between the E. M. F. gen-
erated and the E. M. F.'s necessary to overcome the
resistance and reactance. If, then, three E. M. F.*s are set
up at the istation that are proportional to the above three
E. M. F.*s and bear the same phase relation with regard to
one another, and if these E. M. F.'s are combined in the same
way as the line E. M. F.*s, it is evident that their resultant
will make the voltmeter indicate the E. M. F. at the end of
the line. For example, take the simple case shown in
Fig. 25 (a). ^ is an alternator supplying current to the line.
T^ is an ordinary potential transformer, the secondary of
which gives a voltage proportional to the generator voltage
and in step with it. If the voltmeter V were connected
directly to T\ it would evidently indicate the station voltage,
but what is wanted is an indication of the voltage at the far
end of the line, and in order to get this, the voltage of 7^
must be reduced by an amount equal to the sum of the drops
caused by the reactance and resistance. An adjustable
reactance a and an adjustable resistance d are therefore
inserted in the circuit. The drop through d will be pro-
portional to and in phase with the resistance drop, and the
voltage across a will be proportional to and in phase with the
inductive drop. From the way in which the connections are

Digitized by VjOOQIC

SWITCHBOARDS AND

§25

made, it is easily seen that the voltage acting on Fis a com-
bination of the voltages of 7"^ a, and b. The drop across
a and b will increase as the current in the line increases;

Une

f7j5j?ywwwt

Trarj^rmer.

7i Loait
Urte

(•>

Line

lb Lfioa
Une

XXX

Ct4rref7f
Transformer

:cca;

"^ Ccmpensafor

(h)
Fio. 25

hence, the voltmeter reading will decrease (because the
connections are made so that the pressures across a and b
cut down the E. M. F. applied to V), The voltmeter will.

Digitized by VjOOQIC

§26 SWITCHBOARD APPLIANCES 27

therefore, indicate the true pressure at the end of the line
because both the ohmic and inductive drops are accounted for.
Fig. 25 (a) is intended to illustrate the principle only;
the actual connections are more nearly as indicated in
Fig. 25 (d). , Here A is the alternator, as before, and T^ the
potential transformer. T' is a small current transformer, the
primary of which is connected in series with the line and
the secondary to the compensator proper, which consists of
three parts, a, d, and D. The E. M. F. generated in the
secondary of T^ is proportional to and in step with the gen-
erator E. M. F. The current in the secondary of T is pro-
portional to the load; a is sl non-inductive resistance and d
is a reactance coil wound on an iron core. These coils are
connected in series, and the current supplied from the sec-
ondary of T' passes through them. The E. M. F. across a is
therefore in step with and proportional to the resistance drop
in the line; while that across d is in step with and pro-
portional to the back E. M. F. due to the reactance of the
line. Z> is a small transformer in shunt with a; its secondary
E. M. F. is in step with and proportional to the E. M. F. across
a; b is also provided with a secondary that gives an E. M. F.
in step with and proportional to the E. M. F. across b. All
these devices, i. e., a, b, and Z>, are in one piece of apparatus,
and terminals from the secondaries of D and b are brought
out to two multipoint switches, so that the number of turns
in each may be adjusted to suit different lines. For three-
phase circuits, a and b are supplied from two series-trans-
formers whose primaries are connected in series with two of
the lines and whose secondaries are in parallel. The volt-
meter compensator made by the General Electric Company
operates on practically the same principle.

FUSES AND CIRCUIT-BREAKERS

27. Either fuses or circuit-breakers may be used to pro-
tect the generators or circuits from an excessive flow of
current, due either to a short circuit or overload. Fuses
are not used as much as they once were, as it has
been found that circuit-breakers are more reliable. The

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28 SWITCHBOARDS AND §25

circuit-breaker may be a separate device, or the main
switch may be provided with an automatic tripping device,
as already described.

FUSES

28. A fuse consists of a strip or wire of fuSible metal
inserted in the circuit, and so proportioned that it will melt
and open the circuit if the current for any reason becomes
excessive. Fuses are often made of a mixture of lead and
bismuth, though copper and aluminum are also used.
Aluminum is used very largely for high-tension fuses.

For low-tension switchboards, plain open fuses may be
used; but for high-tension work, it is necessary to have them
arranged so that the arc formed when they blow will not
hold over. Moreover, it is necessary to have high-tension
fuses arranged so that they can be renewed without danger
to the switchboard attendant.

29. Fig. 26 (a) shows a type of fuse block used by the
General Electric Company on alternating-current switch-
boards; (d) shows the shape of the aluminum fuse used in
the block. The fuse holder is made in two parts, the lower
part A being of porcelain and the upper part B of lignum
vitae. The lower part is provided with blades c that fit
between the clips d, d', in the same way as the blades of
a knife switch. These clips lie in slots in the marble
board F and are connected to the line and dynamo by means
of terminals g and h. By adopting this arrangement, the
whole block may be detached from the board by simply
pulling it straight out, thus pulling the blades out of the
clips. The fuse is shown at /, and is clamped by means of
the screws w, n. A vent hole / is provided in the lignum-
vitae cover, and the rush of air through this vent, together
with the confined space, results in the suppression of the arc.
This fuse block is suitable for currents up to 150 amperes at
2,500 volts. For higher pressures fuse blocks are used in
which the fuse is pulled wide apart as soon as it blows, thus
breaking the arc.

The use of the fuses on low-tension lighting switchboards

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§25 SWITCHBOARD APPLIANCES 28

is not as common as it once was, their place being taken by
the automatic circuit-breaker. Fuses are, however, used
considerably on alternating-current boards and also for pro-
tecting individual circuits on low-tension, direct-current
boards. They are not as convenient or reliable as circuit-
breakers, because it takes time to replace them when they
blow, and only too often they are replaced with a heavier fuse
or even a copper wire, which is of scarcely any use as a pro-
tection. Again, fuses of the same size do not always blow at

T~V

(a)

(b)

Pig. 26

the same current, as much depends on the nature of the fuse-
block terminals. If the clamps are not screwed up tightly,
local heating will result, and the fuse will blow with a smaller
current than it should. Also, it has been found that a fuse
of a given cross-section and material will carry a heavier
current when the distance between the terminals is short
than when it is long, on account of the conducting aw^ay of
the heat by the terminals.

30. Fig. 27 shows a type of high-tension enclosed fuse
made by the Stanley Electric Company. The fuse is held in
the holder a, which can be pulled out of the clips d when a

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30 SWITCHBOARDS AND §25

fuse is to be renewed. Suitable blades are provided at each
end to engage with clips b. The clips and connecting studs
are thoroughly insulated by the porcelain insulators c, c,
which prevent leakage of current to the supporting panel d.
The fuse h passes through a fiber tube e and is held at each end
by screws i\ tube e is enclosed in the hard-rubber tube / of
large diameter. At each end of the fuse is a cavity in which

Pio. 27

is placed a carbon ball g, and when the fuse blows the balls
are forced up against the openings leading to the ter-
minals, thus cutting off the arc. These fuses can handle a
current of 50 amperes at 20,000 volts. There is a small
hinged lid k on top that is thrown up when the fuse blows,
and thus acts as an indicator to show which fuse has blown.

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§25 SWITCHBOARD APPLIANCES 31

The high-tension fuse used by the Westinghouse Company
consists of two long hinged wooden arms that are held
together by the fuse against the action of a spring. As
soon as the fuse melts, the arms separate, thus placing a
break of several feet in the circuit and rupturing the arc.

CIRCUIT-BREAKERS

31. Some circuit-breakers have already been described
in connection with high-tension switches. The clpcult-
breakep is essentially an automatic switch that opens the
circuit whenever the current exceeds the allowable limit. It
is therefore intended more as an automatic safety device than
as a switch for regularly opening or closing the circuit.

Pio. 28

Circuit-breakers are made in great variety, handling cur-
rents from a few amperes up to several thousand, and are con-
structed for both alternating current and direct current. In
nearly every case they consist of a switch of some kind that
is held closed against the action of a spring. The main cur-
rent passes through an electromagnet or solenoid, and when
the current for which the breaker is set is exceeded, this
magnet attracts an armature or core and operates a trip,
thus allowing a switch to fly out. In some cases the breaker

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32 SWITCHBOARDS AND §25

opens both sides of the line, though often they are single-
pole and open one side only. We will illustrate here a few
examples to show their general method of operation.

32. General Electric Circuit-Breakers. — Figs. 28
and 29 show a type of General Electric circuit-breaker

designed for 125- or 250-volt circuits. One of the principal
features of this circuit-breaker is the main contact used. It
consists of a U-shaped laminated contact a which is pressed

lirmly against the con-
tacts b, b by means of a
togglejoint, when handle h
is forced down. Each main
contact is provided with a
pair of light auxiliary con-
tacts m, m that can be
easily renewed. These
wipers press against the
carbon blocks p, p, and
when the breaker flies out,
the arc is finally broken
between the carbon blocks
and the wipers. Laminated
contacts are not liable to
stick and they make a very
good contact because of
the firm pressure and the
slight wiping action caused
by the closing of the
breaker. The tripping
coil 5" attracts the arma-
ture A when the current becomes excessive and trips the
breaker, which is promptly opened by the spring /. The
current for which the breaker is set 'may be adjusted by
means of the screw v and the breaker may be tripped by
hand at any time by pulling: down on the knob w. The
breaker shown in Fij^. 2<S is a double-pole; Fig. 29 shows a
similar breaker of the single-pole type.

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§25

SWITCHBOARD APPLIANCES

33. General Electric M K Circuit-Breaker. — This
breaker, Fig. 30, has been very widely used for 500- volt,
direct-current, railway switchboards and is here shown as an
example of the class of circuit-breakers in which a magnetic
field is used to extinguish the arc. In Fig. 30, B is a heavy
tripping coil through which the main current passes. Tht

current enters the coil through the stud A; from the coil it
passes to a connection on the back of the heavy copper con-
tact block C, When the breaker is closed ready for service,
as shown in the figure, the main current passes from C to
the laminated contact D^ D and out to the line through the
heavy block E^ which has a terminal like A in the rear.

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SWITCHBOARDS AND

§25

When the breaker is closed, the hinged iron armature F is
held up by a spring G, the tension of which depends on the
adjustment of a thumbscrew /. Attached to plate F is a
trigger //^ that has a shoulder against which a projection
on the main handle yoke IC bears. To set the breaker, the
main handle L is pulled down hard; this forces Dy D up against
blocks C and E^ and also causes the projection on K to
engage trigger Hy which holds the circuit-breaking parts in
place. In setting the switch, spring il/ is extended. When
the breaker trips, solenoid B draws down armature Fy and
with it trigger Hy which liberates the switch yoke and allows

Pio. 81

the strong spring M to pull down Z>, Z7, and hence open
the circuit at C and E, In order to prevent burning of the
main contacts, a shunt path is provided, as indicated by
the circuit T-S-R-P-O-P-R-S'-Uy Fig. 81. 5, 5* are two
magnetizing coils that set up a strong magnetic field between
the auxiliary contacts P, P. When the breaker is closed,
the contact piece O is pushed up between contacts /*, P
which are pressed firmly against O by springs Qy Q.
When the breaker trips, contact Z>, D leaves Cy E a little

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§25 SWITCHBOARD APPLIANCES 35

before O leaves P, P, so that for a short interval the main
current takes the path through the auxiliary contacts and
blow-out coils 5, 5". A strong magnetic field is thus set up
and when the circuit is finally broken at the auxiliary contacts,
the arc is instantly blown up through an opening in the top
of the breaker. Whatever burning action there may be is
thus transferred to the auxiliary contacts, which are easily
renewed or repaired.

34. Cutter Circuit-Breaker. — Fig. 32 shows the
Cutter (l. T. E.) laininated-tyi>e circuit-breaker. . The

main contact a is lam-
inated and is pressed
against the contact
surfaces by means of
the handle working
through a togglejoint
at^. The tripping coil
is shown at d and
when the current ex-
ceeds the amount for
which the breaker is
set the core inside d
is suddenly drawn up,
thus striking a trig-
ger and allowing the
breaker to fly out.
The position of the
core in d can be
changed by adjusting

^, , Fio. 82

screw Cy thereby vary-
ing the current at which the breaker trips. Auxiliary carbon
contacts h, b do not open until after the main contact so that
the burning action is confined to the carbon contact surfaces.
The Westinghouse circuit-breakers are very similar in
general appearance and operation to the type shown in
Fig. 32, the main difference being in the arrangement of
the tripping coil.

45—24

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SWITCHBOARDS AND

§25

GROUND DETECTORS

35. Ground detectors are used to determine whether
or not a line or conductor, that should normally be insulated,
is in contact with the ground or any conductor leading to
the ground. A voltmeter makes a very good ground

Pio. 88

detector, because it not only indicates whether a ground is
present, but by its deflection it shows whether the path of the
current to ground is one of high resistance or low resistance.

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§25 SWITCHBOARD APPLIANCES 37

In order to indicate grounds, the voltmeter may be con-
nected as shown in Fig. 33 {a). If the line a should be
grounded, as indicated by the dotted line, and the switch
blade placed on point i, no deflection would result. If,
however, the blade is moved to point 2, current will pass
from line a through the ground on the line to the voltmeter
to point 2, and thence to the line d, thus completing the
circuit. When a deflection is obtained on point 2, it shows
that line a is grounded; and when obtained on point i, it
shows that line d is grounded. If the ground is of high
resistance, the deflection
will be comparatively
small; if of low resistance,
the deflection will be large.
In Fig. 33 (a)y the current
will flow through the volt-
meter in the opposite direc-
tion on point 2 from what
it will on point 1; hence, "^

the voltmeter must have ^ P10.34

its zero point in the center of the scale, so that it can read
either way. Voltmeters, however, have their zero point at
the left-hand end of the scale, and it is convenient to have
a switch that will allow the ordinary voltmeter to be used
either as a voltmeter or ground detector. Fig. 33 (d) shows
an arrangement for doing this. When the switch is in the
position 1-1\ the voltmeter F is connected directly across
the line and gives the voltage on the system; when in the
position 3-^\ the voltmeter indicates any grounds, such
as G'^ that may be present on line d. When 5 occupies the
position 2-2\ V indicates grounds on line a, as at G.

36. Another very common arrangement for detecting
grounds is shown in Fig. 34, where two lamps r, d are con-
nected in series across the lines. The voltage for which
these lamps are designed is equal to that of the dynamo, so
that when the two are connected in series, they will burn
dull red. At the point between the lamps, a connection is

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SWITCHBOARDS AND

§25

made to ground through a switch or a push button /. If con-
tact is made at / and there is no ground on either line,
the brilliancy of the lamps will not be altered. If there is a
ground on b, as indicated at G^ lamp d will go out when
switch / is closed, and c will burn brightly. This lamp
detector is simple, and while it serves as an indicator of
grounds, it is not as satisfactory as the voltmeter detector,
as it does not give accurate indications as to the resistance
of the fault.

37. Fig. 35 shows a lamp ground detector suitable for a
three-wire, low-tension system. Three lamps A, /„ /, are
connected in series across one side of the system, and

a ground connection is
,,,,-, , made at x through key

A^. When all three lines
are clear of grounds, the
lamps will bum at a dull
red, they will all be
equal in brightness,
and their color will not
change when key A^ is
pressed. If line C be-
comes grounded at C,
then, when K' is pressed,
/, and /, will go out, and /, will come up to full candlepower.
If a ground occurs at 6^" on line B, lamp /, will go out and
/,, /, will brighten up, but will not come up to full candle-
power because two of them will be in series between B and C.
If there is a ground at G'" on line A, all the lamps will
come up to full candlepower, because they will all get the full
voltage, /, being across A B and /,, /, in series across A C

38. The ground detectors just described, apply more par-
ticularly to low-tension, direct-current installations, but similar
arrangements may be adapted to high-tension, alternating-
current systems by using potential transformers. Fig. 36
shows one method used by the Westinghouse Company
on their alternating-current switchboards. The regular

Qnouncf

Pio. 35

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§26

SWITCHBOARD APPLIANCES

voltmeter F, with which the switchboard is equipped, is here
used also as a ground detector. P is a, plug switch by means
of which points 1 and 2 or 1 and 3 may be connected together.
Under ordinary conditions, the plug is in 1 and 2, thus con-
necting the primary of the potential transformer across the
line, and V serves as an ordinary voltmeter. 5" is a key that
connects one side of the line to ground through the trans-
former primary. If there happens to be a ground on the
side b, as shown at G', the voltmeter will give a reading when
5 is pressed. By placing the plug in points 1 and 3, side a may
be tested. When the key 5 is not pressed, the lever 5 is against
contact 4, so that V is connected as an ordinary voltmeter.

Pio. 96

39. Electrostatic Ground Detectors. — Ground
detectors operating on the electrostatic principle are much
used on high-pressure, alternating-current switchboards.
They have the advantage that they require no current for
their operation and may be left connected to the circuit all
the time, thus indicating a ground as soon as it occurs. They
also give an indication without its being necessary to make
an actual connection between the line and ground, as is the
case with all the detectors previously described. Fig. 37
illustrates the principle of a Stanley electrostatic ground
detector, which is especially adapted to high-pressure, alter-
nating-current lines because the instrument is not in actual
connection with either of the lines. The fixed vanes 1 and 4,

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SWITCHBOARDS AND

§25

2 and 3 are connected together in pairs, as shown. The
movable vane ['is connected to the ground and is held in the
central position shown in the figure by means of small spiral
springs S, The pairs of fixed plates are not connected
direct to the lines, but are attached to plates a, a' of two
small condensers which consist simply of two brass plates,
mounted in hard rubber but separated from each other.
Plates by b' are connected to the lines. When no grounds

Pio. 17

are present, t and 4, 2 and 3 become oppositely charged
by reason of charges induced on plates a^a' by plates
by b\ At any instant the charge on vanes 1 and 4 will
be similar to that on B\ at the same time the charge on
vanes 2 and S will be similar to that on A, The forces
acting on the vane V are therefore equal and opposite.
Now, suppose that line B becomes grounded at G", This
is equivalent to connecting vane V to line B\ V takes up a
charge similar to 1 and i\ hence, it is repelled by 1 and 4

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§25 SWITCHBOARD APPLIANCES 41

and is attracted by 2 and 5, thus giving a deflection. If A
becomes grounded, a deflection in the opposite direction is
obtained. Instruments
of this kind can, of
course, only be used in
places where the pres-
sure is fairly high, a's the
electrostatic forces pro-
duced by charges due to
low pressures would not
be large enough to oper-
ate an instrument unless
it were made much too
delicate to be of prac-
tical use in a light or
power station. In most
electrostatic detectors,
the lines are connected °' ^

directly to the fixed sectors i, 2, 5, 4 and the con-
densers C, C are omitted.

40. Fig. 38 shows an electrostatic ground detector made

by the Wagner Elec-
tric Company. The
fixed quadrants are
shown at a, a, and the
movable vane at h, b.
The quadrants are
connected to the line
wires, and the vane is
connected to ground.
The vane is held nor-
mally in its central
position by means of
a spring, and the
pointer is deflected
^®- ^ whenever a ground

occurs on either line. The principle of action is the

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SWITCHBOARDS AND

§25

same as that of the electrostatic ground detector just

described.

41. Figs. 39 and 40 show
a General Electric, three-
phase, electrostatic ground
detector. - It is practically
three single-phase detectors
combined in one instrument.
When no ground exists, the
three needles point toward
the center. When a ground
occurs on one of the lines, the
two adjacent needles are de-
flected toward the segments
to which the grounded line is
connected. Should a ground
occur on two lines, the needle
between the segments con-
nected to the grounded lines
will be deflected toward the
one having the lower resist-
ance ground and the two
remaining needles will be

Pio. 40

deflected toward the grounded segments.

POTENTIAIi REGUIiATOBS

42. Where a number of feeders are supplied from a single
dynamo or set of bus-bars, it is often necessary to provide
means for raising or lowering the pressure on these feeders
independently of each other. When alternating current is used,
the pressure on the feeders can be easily adjusted by using
potential regulators. These appliances, while not usually
placed on alternating-current switchboards, are so closely con-
nected therewith that they are here described. There are
many types of regulators but they all take the form of a special
type of transformer with the primary connected across the
mains and the secondary in series with one of the mains.

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§25

SWITCHBOARD APPLIANCES

43, Use of Transformer to Raise Voltage. — An

ordinary transformer connected as in Fig. 41 can be used to
raise or lower the primary voltage by an amount equal to
the secondary voltage of the transformer. When the double-
throw switch is in the position indicated by the dotted lines,
the primary is across the mains and the secondary in series
with the lower main, thus adding 100 volts in this case or
subtracting 100 volts if the connections be such that the
secondary E. M. F. opposes the line E. M. F. When
the switch is thrown to the right, the boosting trans-
former is cut out.

10 wv

*-iooo-v-

IfiQQQQQj

■QQQOW.

i-4

noor

PlO. 41

44, Stlllivell Regrulator. — Fig. 42 shows the connec-
tions for a Stillwell regrulator. It operates in the same
way as the transformer in Fig. 41 but the secondary 5 is
provided with a number of taps connected to a switch Mso
that the amount by which the voltage is raised or lowered
can be adjusted. The primary P is connected to a reversing
switch b so that the secondary E. M. F. can be made either
to aid or oppose the primary E. M. F., thus using the regu-
lator either to raise or lower the line pressure. The contact
arm N is made in two parts, connected through a small
reactance coil r, the object being to prevent momentary
short-circuiting of the transformer sections during the
instant the arm bridges over adjacent contact segments. By

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44 SWITCHBOARDS AND §25

following out the connections, it will be seen that the sec-
ondary is in series with the main circuit and the primary
across the circuit, as in Fig. 41.

45. C R Regulator. — The C R regrulator, made by
the General Electric Company, operates in a manner very

Fig. 42

similar to the Stillwell regulator. Fig. 43 shows the general
appearance of the regulator, and Fig. 44 the connections.
The reversing switch operates automatically and is placed in
the secondary circuit, and not in the primary as in the Stillwell

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§25 SWITCHBOARD APPLIANCES 45

regulator. In Fig. 44 the reversing switch is indicated
at the lower part of the figure, and consists of an arm that
is moved by the arm of the
main switch so as to con-
nect a with either c or b. The
windings consist of a primary
and secondary, the former
connected across the circuit,
and the latter divided into a
number of steps, in series
with the circuit. When the
reversing switch and the main
switch arm are in the posi-
tions shown in Fig. 44, the
main current flows through
the whole of the secondary
winding, and the maximum
increase in voltage is ob-
tained. As the dial switch
arm is turned, the sections of
the secondary are succes-
sively cut out as contact is ^'®- ^
made at d,e,{, etc.; when the arm reaches g, the whole of
the secondary winding is cut out, and the voltage sup-

Pio. 44

plied to the feeder is the same as that furnished by the gen-
erator. When the arm is started on a second right-handed

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46 SWITCHBOARDS AND §25

revolution, the reversing switch is shifted automatically,
so that point a is connected with b^ and as the move-
ment of the dial switch is continued to the right, the
sections of the secondary are successively cut in, and the
current now flows through them in the reverse direction to
what it did before. The second revolution, therefore, lowers
the feeder pressure below that of the generator; when the
second revolution has been completed, the switch is auto-
matically stopped. The dial switch is made so that when
the handle is turned, springs are first compressed and the
blade then unlocked by a cam so that it flies from one con-
tact to the next almost instantly. The switch blade is slightly
narrower than the distance between the contacts, so that
there is no short-circuiting of the transformer sections.

46. A number of regulators are in use in which the volt-
age in the secondary is varied by changing the position of
the secondary with regard to the primary, instead of cutting
turns in or out. By having the secondary coil movable, it
can be arranged so that the amount of magnetic flux passing
through it can be varied, thus varying the amount of the
pressure added or subtracted. In other regulators, both the
primary and secondary coils are fixed, and a movable core
arranged so that the magnetic flux passing through the
secondary can be made to vary.

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§25 SWITCHBOARD APPLIANCES 47

PROTECTION FROM LIGHTNING AND
STATIC CHARGES

47, There are sources of danger to electrical equipments
that may arise outside the station and that may cause great
loss unless ample provision is made for protection. Among
these are danger from lightning, danger from static charges,
or other effects commonly referred to as static, and danger
from short circuits caused by either of the former. Damage
from lightning occurs on systems having overhead lines, but
static charges and the damage resulting therefrom can occur
on systems having either overhead or underground lines.

PROTECTION FROM lilGHTNING

48, Damage from lightning is due to an excessive differ-
ence of potential that may exist between the atmosphere and
the earth, and as overhead electrical conductors offer a path
of comparatively low resistance, the atmospheric electricity
will seek such path to the earth, unless prevented by
suitable methods of lightning protection. Any properly
designed piece of apparatus should have sufficient insulation
to withstand a potential considerably higher than that nor-
mally imposed on it, and to produce a ground, a lightning
discharge must cause an excessive rise in the potential of the
circuit. It frequently happens that the weakest point of insu-
lation is at the switchboard or generator, and in the absence of
sufficient protection, great damage will result at the station.

49, Overhead lines are always liable to accumulate a
certain charge of static electricity even if they are not
actually struck by lightning. Long transmission lines should
be well protected against lightning, as they frequently run
through exposed and mountainous country. If these high-
pressure discharges travel along the line and get into the

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SWITCHBOARDS AND

§25

dynamos at the power station, they are almost sure to
puncture the insulation of the machines and cause a bum-out.
To guard against this, lightning arresters should be provided.

50, simple lil^litnln^ Arrester. — The term light-
ning: arrester does not correctly express the use of these
devices, because they do not arrest the discharge coming
in over the line; they merely divert the charge by providing
a path that the lightning will take to ground in preference
to passing into the dynamo and making a path for itself to
the ground by puncturing the insulation of the machine.

A lightning discharge is generally oscillatory in character,
hence it will not pass through an inductive path if an

Line.

nmr^

Line.

Pio.45

alternative non-inductive path is provided for it. The object
of a lightning arrester is to furnish a non-inductive path to
ground and at the same time make provision for suppressing
the arcing that usually follows a discharge. Fig. 45 shows
a line equipped with lightning arresters of the simplest
possible form. The plates 1, 2 are connected to the lines
and are separated by small gaps g, g from plates 5, 3 which
are connected to the ground. The gap in the arrester should
be more easily jumped across by the discharge than the
weakest insulation on the dynamo; otherwise, the discharge
may jump through the insulation to the ground instead of
jumping across the air gap. The air gap must, of course, be
long enough so that the pressure generated by the dynamo

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§25 SWITCHBOARD APPLIANCES . 49

itself will not be able to jump across it. For pressures up
to 500 volts, a gap of iV inch should be suflBcient.

51. Reactance, or Choke, Coils. — In order to force the
discharge to pass through the arrester, clioke colls, react-
ance coils, or kicking: coils, as they are variously called, are
inserted between the arrester and the device to be protected.
Such coils consist of a few turns of wire or

copper strip connected in the circuit as shown
at A, A in Fig. 45. The discharge, in prefer-
ence to overcoming the inductance of these
coils, will jump the air gaps and pass off to
ground. Fig. 46 shows a typical reactance
coil of small size suitable for low-tension
work. Fig. 47 shows a Westinghouse choke
coil made of flat copper ribbon and mounted
on a heavy glass insulator. This coil is for
use on a high-tension circuit; hence, thorough
insulation from the ground is necessary.

52. Suppression of Arcing. — The

simple arrangement of air gaps shown in
Fig. 45 would not be suitable for electric-
light and power circuits for the following reason: If a dis-
charge comes in over both the lines at once, as is quite
likely to happen, because the lines usually run side by side,
an arc will be formed across both the gaps, and current from
the dynamo will follow the arc. This will practically short-
circuit the dynamo, and such a large current will flow that
the plates or contact points of the arrester will be destroyed.
It is necessary, then, to have in addition to the air gap some
means for suppressing or blowing out the arc as soon as it
is formed. It is also necessary that as soon as the discharge
has passed, the arrester will be in condition for the next
discharge. Generally speaking, the arc from a direct-current
machine is not as easily extinguished as that from an
alternator; probably because every time the current passes
through its zero value it loses some of its ability to hold the
arc. In some cases, the arc is broken by being drawn out

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50 SWITCHBOARDS AND §25

until it can be no longer maintained; in others, the air gap
is so placed that it will be surrounded by a magnetic field,
so that when the arc is formed it is forced across the field
and stretched out until it is broken. Another method is to
make the arc occur in a confined space so that it will be
smothered out. Still another method is to make the cylinder
or plates between which the arc jumps of a so-called non-
arcing metal, the vapor of which
offers a high resistance to the
discharge. Some arresters will
work on either direct or alter-
nating current; but, generally
speaking, the arrester has to be
selected with reference to the
voltage of the circuit on which it
is to be used and also with refer-
ence to the kind of current.

53. Ground Connections
for LiiKhtning^ Arresters.

Arresters will be of little or no
use if good ground connections
are not provided for them. The
following methods of making
ground connections are recom-
mended by the Westinghouse
Company: A ground connec-
tion for a line or pole lightning
F^^-*7 arrester is shown in Fig. 48.

A galvanized-iron pipe is driven well into the ground and the
top of it surrounded by coke, which retains moisture; the wire
is run down the pole and connected to the top of the pipe as
indicated. The wire is sometimes incased in galvanized-iron
pipe for about 6 feet from the base of the pole and if this is
done, it is well to solder the ground wire to the pipe at a.
The following method of making the ground connections at
the station is recommended: A hole 6 feet square is dug
5 or 6 feet deep in a location as near the arresters as possible.

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§ 25 SWITCHBOARD APPLIANCES 51

preferably directly under them. The bottom of this hole is
then covered with charcoal or coke (crushed to about pea size)
to a depth of about 2 feet. On top of this is laid a tinned,
copper sheet, about 5 feet by 5 feet, with the g^round wire
(about No. 0 B. & S.)
soldered completely across
it. The plate is then cov-
ered with a 2-foot layer of
coke or charcoal and the
remainder of the hole filled
with eal'th, running water
being used to settle it.
This will give a good
ground, if made in good,
rich soil; it will not give a
good ground in rock, sand,
or gravel. Sometimes
grounds are made by put-
ting the ground plate in
a running stream. This,
however, does not give as
good a ground as is com-
monly supposed, because
running water is not a par-
ticularly good conductor
and the beds of streams
very often consist of rock. „ ^

When lightning arresters

are installed, all wires leading to and from them should be as
straight as possible. Bends act more or less like a choke
coil and tend to keep the discharge from passing ofiE by way
of the arrester.

ARRESTERS FOR DIRECT CURRENT

54. Garten Arrester. — Fig. 49 illustrates the Garten
arrester. The discharge points are of carbon, shown at h
and y. These are about aV inch apart, and the lower one is
connected to ground; / is a coil of wire wound on the tube^,

45—26

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62 SWITCHBOARDS AND §25

closed at the top; ^ is a small core of iron attached to the
rod d, which in turn connects, by means of a small flexible
cable, to one end of a resistance h. The other end of the
coil connects to the other end of the resistance, to which the
line also connects. The resistance b
is made up of a stick of graphite,
which, having practically no induct-
ance, offers little or no opposition to
the discharge and is used to limit the
rush of current that follows the dis-
charge. The discharge comes in over
the line to a^ passes through b to the
rod d^ thence to the carbon point hy
and jumps the air gap to the ground.
The discharge is followed by current
from the dynamo, and, since the coil
is in shunt with the resistance, part
of the current will flow through the
coil, thus drawing up the core e and
breaking the arc between e and h.
The fact that the arc also takes place
in the enclosed tube tends to put
^^' ^^ it out. As soon as the discharge has

passed, the core drops back and the arrester is ready for
the next discharge. This arrester can be used on either
direct- or alternating-current circuits.

55. Westlnffhouse Arrester. — Fig. 50 shows a West-
Ingrliouse arrester used on direct-current circuits. It has
no movable parts, and the arc is extinguished by smothering
it in a confined space. Two terminals b, b are mounted on a
lignum-vitae block and are separated by a space somewhat
less than \ inch. This space is crossed by a number of
charred grooves, so that although the resistance in ohms
between the terminals is very high, the lightning will
readily leap across the space. The block A is covered by a
second block, not shown in the figure, that excludes the air
and confines the arc to the small space between the terminals.

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§25

SWITCHBOARD APPLIANCES

When the arc tends to follow the discharge, the small space
is soon filled with a metal-
lic vapor that will not
support combustion. It
should be noted that this
arrester is intended for
use on direct-current cir-
cuits only, where the pres-
sure does not exceed 600
or 700 volts.

56, General Elec-
tric Arrester. — In the
General Electric ar-
resters for direct cur-
rent, the arc is blown out
by making it occur in a
magnetic field provided ^^ ^

by an electromagnet.

Fig. 51 shows a direct-current arrester with the cover
removed; the case and cover are made of porcelain. The

(a) Pio. 61 id)

part id) holds the blow-out coil^ with its polar projections k, h\

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SWITCHBOARDS AND

§25

r is a gjaphite resistance for limiting the current. The
electrodes are mounted in the cover and are held by
clips k, k!\ the air gap a is about .025 inch in length.
When the cover is in place, clips k\ k! make contact with
the tongues k, k, and give the scheme of connections
shown in Fig. 52. Here a represents the air gap, shown
also at a, Fig. 51 (a), x y is the blow-out coil, r r' the
graphite resistance. The ground connection is made to
the lower end / of the resistance, and the line is connected
to the upper electrode. The terminals of the blow-out coil

TbUne^

resisfoncti

Fio. 52

connect to z and p, so that the coil is in parallel with a por-
tion of the resistance. When a discharge comes in over the
line, it jumps the air gap and passes to the ground through
the resistance, and when the current follows the discharge,
part of it passes through the blow-out coil. When the
cover is placed in position, the air gap a falls between the
pole pieces h, h, and the arc is blown out through an open-
ing in the cover. A portion of the resistance t* is in series
with the coil and spark gap, and thus limits the amount of
current that tends to follow the discharge. The ordinary
type of this arrester is suitable for any direct-current circuit
using pressures of 850 volts or less.

ARRESTERS FOR ALTERNATING CURRENT

57. Westiiighouse Arrester ifor Alternating: Cup-
rentl — Fig. 53 shows a type of arrester that has been largely
used by the Westinghouse Company on alternating-current

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§25

SWITCHBOARD APPLIANCES

circuits. It is known as the Wurts non-arcing arrester, and
consists of a number of milled cylinders a, a separated from
each other by small air gaps. The end cylinders are con-
nected to the lines and the middle cylinder to the ground.
With this arrangement, a single arrester does for both sides
of the line; where, however, the line pressure is high, a
separate arrester is used for each side; and for very high
pressures, such as are used on long-distance lines, a number
of arresters are connected in series. When a discharge
comes in over the line, it jumps the gaps between the

Fio. 53

cylinders and passes to the ground. It is claimed that
the arc does not hold over, because the gases formed by
the volatilization of the metal will not support an arc. The
cylinders are made of what is known as non-arcing metal.
Others claim that the suppression of the arc is due to the
cooling effect of the cylinders and the alternating nature of
the current. These arresters should be examined from time
to time and the cylinders rotated slightly so that they will
present fresh surfaces to each other.

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66 SWITCHBOARDS AND §25

58, General Electric Arrester for Alternating
Current. — Fig. 54 shows an arrester used by the General
Electric Company for alternating-current circuits. It is
somewhat similar to the Wurts arrester, except that fewer
spark gaps are used and a non-inductive resistance r is
inserted in the circuit in order to limit the current following
the discharge. The spark gaps a, a are between the heavy
metal cylinders by b, b, the middle one of which is connected
to ground in the double-pole arrester shown. This arrester,

like the previous one, is not suitable
for use on direct-current circuits.

The arresters just described have
been shown as arranged for indoor
use in the station. They may, how-
ever, be used on the line, in which
case they should be mounted in a
weather-proof box made of iron or
wood. The connections to and from
the arresters should be made with
wire not less than No. 4 B. & S.

59 • Westinghouse Arrester
for High-Tenslon Liines. — When
lightning arresters are used on high-
tension lines, they usually consist
of a number of air gaps connected
in series between the line and the
^'®' ^ ground, the total length of air gap

being so proportioned that the normal voltage of the
system, even if one line becomes grounded, will not cans© a
current to jump across the gaps; the gaps are generally used
in connection with a resistance that will prevent a rush of
current after a discharge. A choke coil is also used to
choke back the electrostatic wave passing along the line, and
make it take the path to ground. Fig. 65 shows one of the
air-gap units used with Westinghouse high-tension lightning
arresters. It consists of seven knurled cylinders a, a^ sepa-
rated by six ^V-inch air gaps, and made of non-arcing metal

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§25

SWITCHBOARD APPLIANCES

The cylinders are arranged so that they can be revolved
in the porcelain holders b^b in case the parts facing each
other should be burned by the discharge.

Fig. 56 shows the connections of a Westlnfirlionse low-
equivalent arrester as arranged for a 6,000-volt circuit.
The line to be protected
is connected at point A.
Two sets of gaps B and C
are connected in series
and to the ground through
a series-resistance-^'. The
gaps C are shunted by a
resistance R and are known
as shunted gaps; gaps B are
called series-gaps. When
the potential at A rises to
an abnormal amount due
to a lightning discharge or other cause, a discharge leaps
across the series-gaps B, If the discharge is heavy, it
will meet with a large amount of opposition in the resist-
ance R^ and will pass over gaps C and resistance R' to
ground. The current that tends to follow the discharge
and that is maintained by the dynamo will take the path

Pig. 65

To Un9

Fig. 56

Sroanc^^

through R instead of passing across gaps C, so that the
effect of the shunted resistance is to withdraw the arc from
gaps C and at the same time cut down the volume of current
so that the series-gaps can suppress the arc. By using this
arrangement a smaller number of gaps at B is needed than

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68 SWITCHBOARDS AND §25

would otherwise be necessary. The series-resistance R^ is
used to limit the initial flow of current and prevent burning
of the cylinders B,

Fig. 67 shows the arrangement of one of these arresters
with its choke coil. The spark gaps are at a, a, while the

T0 Afp0f^ttAX

PIO.S7

resistances are mounted in suitable holders by b. The arrester
shown in Fig. 57 is for 8,500 volts. For arresters of higher
voltage than this, the series-resistance is not mounted on the
same panel with the other parts, but is placed separately on
suitable columns that provide thorough insulation.

60, In the selection of lightning arresters the following
points should be kept in mind:

1. The width and number of spark gaps should not be so
great as to require the potential of the lightning charge to

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§25 SWITCHBOARD APPLIANCES 59

be as high or higher than the potential necessary to rupture
the insulation of the system.

2. On account of its nature, a lightning arrester is
evidently exposed to severe potential strains; consequently,
all live parts must be well insulated. On arresters for low
voltages it is not a difficult matter to secure proper insulation,
as the construction of the arrester itself affords protection.
On high-tension arresters, however, proper insulation is a
more difficult matter.

3. The general design and construction of the arresters,
together with the necessary adjuncts, should be such as to with-
stand very heavy lightning discharges without destruction.

4. As current is apt to follow the slightest discharge, it is
necessary that the arrester should be designed to break the
arc quickly without permitting an excessive flow of current.

5. Line terminals should not be exposed in arresters in
such a manner as to permit of the accumulation of dust, dirt,
bugs, cobwebs, etc., which may facilitate the formation of
short circuits and resulting arcs across terminals.

6. Arresters should be designed to handle heavy dis-
charges of atmospheric electricity without permitting the
same to follow the circuit and puncture the insulation of the.
station apparatus.

61. The importance of adequate protection becomes
greater with the increased extension of the system, for the
reason that the larger systems encounter different atmos-
pheric conditions by extending over greater areas, and the
possibility of trouble increases, also the amount of possible
damage resulting from breakdowns. Thunder storms that
may occur miles distant might be unknown at the station
except for the snapping of the arresters or some sudden
discharge.

The object should be to select the best method of pro-
tecting the system and then to apply a sufficient number of
lightning arresters judiciously located in suitable positions
on the system to prevent absolutely any disruptive discharges
from entering the station and damaging the apparatus.

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60 SWITCHBOARDS AND §26

Special sets of arresters should be connected immediately
outside of the station. On account of the extreme sudden-
ness of the surges caused in the line by lightning discharges
and other static disturbances, the gaps of the arrester, and
ground connection also, must be able to discharge electricity
very freely, in fact more rapidly than it appears on the line;
otherwise, a dangerous rise of potential on the line will not
be prevented.

INSTALI^TION OF ARRESTERS

62. Before arresters are installed, the characteristics of
the surrounding territory should be carefully studied, and if
possible, statistics obtained regarding the frequency and
severity of atmospheric electrical disturbances. The informa-
tion obtained may be somewhat of a guide as to the amount
of protection necessary.

63. liocatlon of Ai-resters. — As regards the location
of lightning arresters, electric systems may be divided into
two groups:

1. Systems in which the individual pieces of apparatus,
such as transformers, motors, arc lights, etc., are many in
number and widely scattered. In these cases lightning
arresters should be located at a number of points for the
purpose of protecting the whole line; they should be more
numerous on the parts of the line particularly exposed, and
fewer in number on the parts that are naturally protected,
especially those parts shielded by tall buildings or numerous
trees. Special efforts should be made to protect the station
by connecting sets of arresters on each line and causing the
discharge to pass to ground before it enters the station. No
definite statement can be made as to the number of arresters
needed per mile, as the requirements will vary widely
according to atmospheric disturbances in the locality.

2. Systems in which the apparatus is located at a few
definite points, as on a high-tension transmission line. In
such cases the arresters should, in general, be located to
protect especially those points where apparatus is situated;

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§26 SWITCHBOARD APPLIANCES 61

that is, should be placed with the object of protecting the
apparatus rather than- the line as a whole. Where circuits
are part underground and part overhead, sets of arresters
should be connected at the points of entrance to and exit
from the underground system.

When determining the safest method of mounting and
insulating the arresters, it should be estimated that all parts
of the arrester except the grounded end of the series-resist-
ance may be momentarily at line potential during the dis-
charge; therefore, the necessity of extra insulation becomes
self-evident.

Two high-tension arresters attached to different line
wires should not be placed side by side without either a
barrier or a considerable space between them. It is prefer-
able to place them on different poles.

PROTECTION BY CONTINUOUS DISCHARGE

64, For overhead systems, excellent protection has been
secured by the placing of barbed wires on the pole lines
above the lines used for distribution; the barbed points serve
to collect the electricity, and the barbed wires should be
thoroughly grounded, at least as frequently as every three
or four poles. An easy method of doing this is to put a
copper plate under the base of the pole, having the ground-
wire connection soldered on the plate and stapled along the
surface from the base of the pole to the top, where it i& con-
nected to the barbed wire. The effect of this sort of protec-
tion is to discharge the atmospheric electricity silently and
continuously, and this method under severe test has proved
successful over large areas, with systems reaching from
30 to 50 miles or more from the station.

Fig. 58 shows the principle of the Westinghouse tank
arrester, a type that has been much used on street-railway
circuits where one side of the system is grounded. The
arrester is connected to the series of choke coils S by closing
plug switches /r, A', A'. The arrester consists of tanks T, T, T
containing carbon electrodes ^, ^ , c; the line is attached at L

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SWITCHBOARDS AND

§25

and the other end of the choke coil goes to the dynamo or
line bus-bar. A circulation of running water is maintained
through the tanks and there is thus a continuous non-induc-
tive path of high resistance to ground for any charges that
may accumulate on the line. The water has such a high

I To Drfftfi,
Fio. 58 •

resistance that the leakage of dynamo current to ground is
not large. There is some leakage, however, and this type of
arrester is only connected to the system during thunder storms,
but while connected it affords very efficient protection.

PROTECTION FROM STATIC CHARGES
65. Static Effects on Hlgh-Tenslori Systems. — It

has been found on systems where high pressure is used that
under certain circumstances, parts of the system may be
subjected to pressures very much higher than the normal.
These effects, for want of a better name, are spoken of as
being due to "static.'' They may be caused by any sudden
change in the E. M. F. of the system, as, for example, when
a dead circuit is suddenly connected to live bus-bars, when a
transformer is switched on to a circuit, when a circuit is
suddenly cut off from the bus-bars, etc. These effects are
not due so much to the static charges themselves, but to the
fact that when a device is switched on to a live circuit, a
current wave at once tends to pass through the device, and
if this wave meets with opposition, pressures much higher
than the ordinary pressure of the system may be set up.
This is somewhat analogous to the case where a current of

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§25 SWITCHBOARD APPLIANCES 63

water is flowing rapidly through a pipe. There will be a
certain pressure on the walls of the pipe due to the head of
water, and this pressure will be practically constant. If,
however, the flow of water be s'topped by suddenly closing
a valve in the pipe, the pressure will for an instant rise to a
very high amount, producing the well-known water-hammer
effect. These sudden rises in pressure on high-tension cir-
cuits may result in puncturing the insulation of transformer
coils, armature coils, cable insulation, or other parts exposed
to the high pressure. Take the case where a transformer is
suddenly connected to a source of high E. M. F. The wind-
ings tend to become charged instantly, but owing to the
self-induction of the coil the current wave that tends to enter
is choked back and a pressure may be set up between the
various layers of the winding that is very much higher than
the normal, thus tending to cause a breakdown. To overcome
these bad effects, a choke coil may be inserted in series with
the device to be protected. This coil chokes back or flattens
out the wave, and allows the pressure applied to the device
to rise gradually. The choke coil must be heavily insulated,
and large enough to flatten out the wave so that the
latter will not injuriously affect the device to be protected.
This means that the coil must be large, and it is difficult to
insert a large choke coil in the circuit without causing a
considerable waste of energy and drop in voltage. Another
method of protection is to use a choke coil in combination
with a spark gap that will break down whenever the pressure'
rises above a predetermined amount. This arrangement is
practically the same as a lightning arrester, and a number of
large plants have their lines fully equipped with lightning
arresters even though the distributing lines are entirely
underground and hence safe from lightning discharges.
The lightning arresters are in such cases installed to
protect the cables against abnormal pressures caused by
the so-called static effects.

66. Static Interrupter. — In some cases, especially on
high-tension lines operating at pressures higher than 16,000

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64 SWITCHBOARDS AND §25

or 18,000 volts, a device known as a static interrupter is
installed to protect large transformers and other apparatus
from the high pressures mentioned above. Fig. 59 shows
the essential parts of the device as made by the Westing-
house Company; one line only is shown in the figure, but
it is necessary, of course, to place one of the interrupters in
each line. A is a. choke coil and B the primary coil of

B a transformer jDr the

(^OOOOOtft)] winding of other appa-

r 00000001

Line

ratus to be protected;
C is a condenser con-
nected between A and
B\ the other terminal
of C is connected to
ground through a
^>w^ fuse D. If the primary

^o- ^ coil B were suddenly

switched on to a live line without the interposition of A or C,
a very high potential would at once be developed at point Zf,
because the current wave could not penetrate the layers of the
winding instantly. The coil A retards the wave, and further-
more the condenser C having a large capacity compared with
the coil B, takes up a considerable portion of the charge, thus
reducing the potential of E for the time being and allowing
the charge to progress well through the coil before the
pressure at E rises to the full amount. In other words,
the condenser C acts in much the same manner as an air
chamber used on a water pipe to prevent the shock due to
a water hammer. By using the condenser in conjunction
with the choke coil, a much smaller coil is sufficient than if
the coil were used alone, and it can thus be designed so that
it will not insert an objectionable amount of resistance or
inductance in the circuit. In practice, the coil A and con-
denser C are mounted together in a case filled with oil, so that
the interrupter has about the same appearance as an ordinary
oil-insulated transformer. The interrupters are connected
directly to the apparatus to be protected so as to practically
form part of the apparatus, because they must be so situated

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§25 SWITCHBOARD APPLIANCES 65

that they will come between the device to be protected and
the source of static disturbance, as, for example, a high-
tension switch.

Overhead systems will naturally be equipped with light-
ning arresters and these will serve to a considerable extent
as protection against static discharges. Underground systems
carrying current at high potential are liable to accumulation
of static charges that may cause a rupture of the cable insu-
lation. Assuming that alternating current of high potential
is transmitted through an underground system, it will be
found that there is a static charge developed in the cable
covering or, under some conditions, in the conduit ducts.
Certain types of conduit have been found to develop con-
denser capacity under these conditions. A 6-foot section of
3-inch, creosoted, pump-log conduit was tested for capacity
with an insulated wire drawn through it and connected in
circuit with a high-potential current. In the darkness, a
faint blue light could be distinguished on the interior surface
of the duct. When circuits are quickly opened, the cable
tends to set up violent oscillations of the system, and the
resultant static potential is liable to rupture, at its weakest
point, the insulation of the cable. Static charges are also
liable to accumulate on generators and switchboard appara-
tus. Electrostatic ground detectors should be used to show
the appearance of any static charge on the line, and on which
particular conductor it may be located.

FIEIiD RHEOSTATS
67. Field rheostats are inserted in the field circuits
of the generators in order that the voltage may be adjusted
by varying the field strength. The rheostat must therefore
be able to carry the field current continuously without over-
heating. The resistance of the rheostat will depend on the
resistance of the field winding with which it is used, and the
range of voltage variation desired. Very often the rheostat
has a maximum resistance about equal to that of the field,
though in many cases it is not necessary to have as much as

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66 SWITCHBOARDS AND §25

this. Field rheostats are made in a great variety of styles
and sizes suited to various classes of machines. They are
also constructed for various methods of mounting, but all
consist of a suitable resistance connected to a multipoint
switch of some kind so that the amount of resistance in the
field circuit can be varied. Small or medium-sized rheostats
are generally mounted on the rear of the switchboard and
operated from the front by a hand wheel. For large rheostats
the resistance can be separate from the board with leads

Pio. 00

running to the switch located on the back of the board, or the
switch can be mounted with the resistance and be operated
from the switchboard by means of chain and sprocket wheels,
or from a pedestal, with a hand wheel, placed in front of the
board. Either of the latter methods are preferable to run-
ning leads from the resistance to the board, because quite a
number of wires are required and there is danger of some
becoming broken. In very large stations, the rheostats are

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§25

SWITCHBOARD APPLIANCES

often bulky and must be placed quite a distance from the
switchboard; in such cases the rheostat switch is moved by
means of a small motor controlled from the switchboard.

68* Fig. 60 shows a General Electric field rheostat

of a type much used for 500-volt railway switchboards. The
rheostat is mounted on the back of the board and operated
by the hand wheel IV in front. The resistance wire or strip
is wound on asbestos tubes that are afterwards flattened and
clamped between pieces of sheet iron covered with asbestos,
the iron strips serving to conduct the heat from the wire.
In rheostats of large capacity, the resistance is in the form
of cast grids. Fig. 61 shows the connections for the
rheostat, Fig. 60. A
small resistance c is
connected to the contact
rings ^, d and contacts
a, a'. When the arm is
in a position where a, a'
are on adjacent con-
tact points, resistance c,
which is equal in amount
to the resistance be-
tween the rheostat con-
tacts, is in parallel with
the resistance between
the contacts. Thus, t>y P'^- «^

using resistance r, the change in resistance due to a move-
ment of the arm from contact to contact is one-half what it
would be if no auxiliary resistance were used. The varia-
tions in field strength are, therefore, as gradual as in an
ordinary rheostat using. twice the number of contacts.

69. Field STvitclies. — Field switches are used to open
the field circuits of dynamos and they are, therefore, of com-
paratively small current-carrying capacity. Field windings,
particularly those of large alternators or high- voltage, direct-
current machines, have a high inductance, and if the circuit
is suddenly opened very high E. M. F.'s may be induced,

45—26

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SWITCHBOARDS AND

§25

^/e/cf JVy/Vto^.

sufficient in many cases to break down the field insulation.
It is therefore necessary, with such machines, to arrang^e the
field switch so that when the field circuit is broken, a path
is at the same time established throug^h a dischargee resist-
ance. This allows the induced E. M. F. to set up a current
through the local circuit thus provided, and strain on the
windings is avoided. Fig. 62 shows a common arrangement
of field switch and discharge resistance as used for 500-volt
street-railway generators. The tongue / is wide enough to
bridge over the gap between the con-
tact segments a, a' of the switch 5,.
which is shown in the position that it
occupies when the generator is in
operation. The current then passes
through the field rheostat r and the
switch 5, as indicated by the arrow-
heads. When the switch is moved to
the position indicated by the dotted
line, connection between the field and
the negative side of the armature is
broken, but before the break takes
place, tongue / comes into contact
with a', so that the shunt field, the
rheostat r, discharge resistance r\
and pilot lamp / all form a closed cir-
cuit. The shunt field is thus able to
discharge through this closed circuit.
When the machine is being started,
the tongue / is placed in its mid-
position, so that current can flow through r' and / as
well as through the shunt field and rheostat r. As the
machine builds up, the pilot lamp becomes brighter, thus
giving the attendant an indication as to whether the machine
is "picking up*' properly or not. After the machine has
come up to voltage, the switch is moved to the position
shown in the figure and the pilot lamp is cut out. On some
boards, five or six lamps in series are used in place of the
resistance r' ^ud the single lamp /. Another type of field

Lamp-

Ser/esf/e/t/.

Pio. 63

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§25 SWITCHBOARD APPLIANCES 69

switch with field-discharge resistance, as used in the exciting
circuit of alternators, is shown in Fig. 73.

70. Recording Wattmeters. — Well-equipped switch-
boards are generally provided with one or more recording
y^attmeters, to record the output, in kilowatt-hours, of
each machine or of the station as a whole. Readings of the
total output are very valuable in making tests on the effi-
ciency of the station and in keeping track of the cost per

Fio.flS

kilowatt-hour. Sometimes it may be desirable to know the
output of individual machines, but usually a knowledge of
the total output is sufficient and a single total output record-
ing meter is installed, as shown at 11, Fig. 65.

Fig. 63 shows a Tliomson recording wattmeter for
use on direct-current switchboards. These meters have to
carry large currents, hence their construction differs some-
what from the ordinary Thomson meter, though the principle

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70 SWITCHBOARDS AND §25

of operation is the same. The series-coils of the ordinary
meter are here replaced by the heavy copper bar a,
through which the current passes, connection being: made on
the back of the board to the lugs b, b. Above and below
this bar are the two small armatures c, c, which are con-
nected, in series with a resistance, across the line, so that
the current in them is proportional to the voltage. Cur-
rent is led into the armatures through a small silver
commutator d, as in the ordinary recording meter, and
the reading is registered on a dial e in the usual way.
The damping magnets used to control the speed are con-
tained in the case /. The main current flowing through the
crosspiece a sets up a field aroimd the crosspiece, and this
field acts on the two armatures ^, c. This instrument is
constructed so that outside magnetic fields have little or no
influence on it. In some of the older styles of meters, the
magnetic field surrounding the heavy conductors on the back
of the board affected the meter. In this meter any stray
field, affects both the armatures Cy c, which are so connected
that an outside field tends to turn them in opposite direc-
tions, and the disturbing effect is thus neutralized. The
field set up by the instrument itself is in opposite directions
on the upper and lower sides of <z, so that these two fields
propel the armatures in the same direction. For alternating-
current boards, total-output recording meters of the induction
type are used.

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§25 SWITCHBOARD APPLIANCES 71

SWITCHBOARDS

71. The switeliboard is a necessary part of every plant.
Its object is to group together at some convenient and
accessible point the apparatus for controlling and distributing
the current, and the safety devices for properly protecting
the lines and machines. Scarcely any two switchboards are
alike in every particular; their layout and the type of
apparatus used on them depend on the character of the
system used, the number and size of dynamos, the number
of circuits supplied, etc.

72. General Construction. — Switchboards were for-
merly made of wood and consisted simply of a built-up board
or wall sufficiently large to accommodate the instruments.
This construction was objectionable on account of the fire
risk, and the only type of wooden board now allowed by the
Fire Underwriters consists of a skeleton frame of well-
seasoned hardwood filled and varnished to prevent absorption
of moisture. A skeleton board of this kind is cheap and is
suitable for those places where the expense of a slate or
marble board is not warranted. Modem boards are nearly
always made of slate, marble, soapstone, or brick tile. Slate
is usually satisfactory for low-tension work, but it should pe
avoided on high-tension boards, because it is liable to contain
metallic veins. A good quality of marble is the material
generally used for modem boards. The slabs making the
boards may vary from } inch to 2 or 2i inches in thickness,
depending on their size. Most central-station slate or
marble boards are made 2 inches thick with a bevel around
the edge of i or f inch. They are supported by bolting to
angle irons i, i. Fig. 64, and are stood out from the wall by
means of braces b,b. Station boards built up as shown
in Fig. 64 are usually about 90 Inches high. It has become
customary to build up boards in panels* each panel carrying

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72 SWITCHBOARDS AND §25

the apparatus necessary for a generator or one or more
feeders. Those carrying the instruments for the generators
are known as generator panels; those carrying the instru-
ments for the feeders, as feeder panels. This system
allows the board to be easily extended as the plant grows in
size, as panels can be added to those already in use. The
extra panels are attached as indicated by the dotted lines in

Fig. 64

Fig. 64, the panels being held together by means of bolts
passing through holes h in the angle irons. For high-pressure
boards using over 8,000 volts, the marble should be polished
on both sides in order to secure better insulation. Also, if
oil switches are mounted on the back of the board, the mar-
ble should be coated with varnish or similar substance to
prevent absorption of oil.

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§26

SWITCHBOARD APPLIANCES

DIRECT-CURRENT SWITCHBOARDS

73. Railway Switchboard. — Fig. 65 shows a typical
dlrect-ciiprent switchboard as arranged for street-railway
operation on the ordinary 500- volt rail-return system. The
board consists of three generator panels A, A, A^ one total-

output panel B, and five feeder panels C, C, etc. One of the
generator panels is left blank to provide for a future gener-
ator. Each generator panel is equipped with -|- and — main
switches i, i, voltmeter plug 2, field switch 3, pilot-lamp

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74 SWITCHBOARDS AND §26

receptacle 4y field rheostat (operated by handle 5), machine
ammeter 5, and machine circuit-breaker 7. The total-output
panel carries a voltmeter 9 that can be connected to either
machine by means of the voltmeter plug, a total-output
ammeter 10 that indicates the combined current output of
the generators; recording wattmeter 11 records the total
output in kilowatt-hours. Each feeder panel is equipped
with a single-pole feeder switch 12, a feeder ammeter 13, and
a feeder circuit-breaker 14. Since on a groimd-retum railway
system the current retmns through the rails, which are
connected to the negative bus-bar, the feeders are connected
to the positive bUs-bar only, hence single-pole feeder
switches are used.

Fig. 66 shows the connections for the board. Two feeder
panels only are shown and the instruments and switches are
numbered to correspond with Fig. 65. If lightning-arrester
reactance coils are used on the switchboard, they will be
inserted as indicated on the left-hand feeder panel. The
equalizer switches are mounted on pedestals near the gener-
ators and the equalizer connections are not brought to the
switchboard. When the voltmeter plug is inserted in either
receptacle, terminals a and c, b and d are connected, thus
placing the voltmeter across either machine; the voltmeter
connections are made at the lower terminals of the main
switch, or **back'' of the switch, so that voltmeter readings
can be taken before a machine is thrown in parallel by
closing the switch.

74. lii^litln^ OP Power Switcliboapd . — Fig. 67 shows
connections for a simple two-wire board suitable for two
generators and three two-wire feeders. Three bus-bars are
provided, the equalizer bar being mounted on the board.
Bach generator panel has a machine ammeter a connected
across ammeter shunt s, circuit-breaker b, voltmeter plug c,
main switches d, field rheostat e, and pilot lamps h, h. As
this board is intended for low pressure, 110 to 250 volts,
field switches and field-discharge resistances are not pro-
vided. A total-output ammeter M is connected between the

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§25 SWITCHBOARD APPLIANCES 75

Pio.e7

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76 SWITCHBOARDS AND §25

generator and feeder panels to indicate the combined current
output of the generators; voltmeter V indicates the voltage
of either machine. Each feeder panel is equipped with a
feeder circuit-breaker g and feeder switch /. The lamps k^ I
may be connected either across the bus-bars, as shown for /,
or to the feeders, as at k. In the latter case the lamp will
go out when the circuit-breaker of the corresponding feeder
trips, and the lamp thus serves as a circuit-breaker telltale.
If a lamp ground detector were used on the board, it would
be connected as shown by the dotted outline at D.

In large stations there are, of course, a large number of
generator and feeder panels on the switchboard. This
increases the size of the board, but each generator or feeder
added merely repeats the connections of the other panels
and no new features are involved.

AliTERNATING-CURRBNT SWTTCHBOABDS

75. The arrangement of ordinary alternating-current
boards is, in many respects, similar to that of direct-current
boards. They are usually built up in panels in the same way
as the boards previously described. Owing to the fact that
alternators are generally separately excited, the switchboard
contains some extra apparatus connected with the exciter
that is not found on direct-current boards. The wiring and
connections will also depend on whether single-phase or
polyphase alternators are used.

76, single-Phase Generator Panel. — Fig. 68 (a) and
{b) gives front and rear views of a typical alternating-current
panel for one single-phase generator. Such a board would be
used where only one single-phase machine is operated on a
single line, and represents about the simplest possible
arrangement. This panel is equipped as follows: Main
switch a, electrostatic ground detector ^, voltmeters, ammeter
d, voltmeter switch <?, field switch /, generator rheostat g^
exciter rheostat h, main fuses ^, and potential transformer/.
The main switch a is of the quick-break type and is provided
with the marble barrier / between the blades to prevent arcing

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§25 SWITCHBOARD APPLIANCES 77

across. The switch / is used to disconnect the field of the
alternator from the exciter and is provided with auxiliary car-
bon contacts to prevent burning at the blades. The rheostat^
is mounted on the back of the board and is operated by a
hand wheel in front. This rheostat is connected in series
with the field of the alternator, so that the field current may

Pig. 68

be adjusted. The rheostat h is in the shunt field of the exciter
and serves to regulate the exciter voltage. Sometimes the
rheostat g is not used, the field current of the alternator being
increased or decreased by raising or lowering the exciter volt-
age by means of the rheostat h. It is best, however, to have
the rheostat g also, especially if two or more alternators are

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SWITCHBOARDS AND

§25

excited by the same exciter, because it then allows the field
current of each alternator to be adjusted independently of the
others. The voltmeter c is connected to the machine through
the potential transformer /, and a small voltmeter switch e

ExctfeK

PlO. 09

is somtimes placed in circuit so that the instrument may be
cut out of circuit when not needed. The main fuses k are of
the enclosed type. No synchronizing device is needed on
this board, as it is intended for a single machine only.

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§26 SWITCHBOARD APPLIANCES 79

77. The rear view of the board will gfive a good idea as
to the way in which the wiring is arranged. Heavy rubber-
covered wire should be used for this work, and especial care
should be taken to see that everything is thoroughly insu-
lated and neatly done. The leads from the alternator
connect to terminals 1 and 2, and the line connects to ter-
minals 3 and 4. The potential transformer / used to lower
the pressure for the voltmeter, is mounted on an iron frame-
work at the base of the board, and when the lightning
arresters are placed on the board, they are usually mounted
on a similar framework rather than on the back of the board
itself. This makes them stand out so that they do not crowd
the wiring on the back. Fig. 69 shows the general scheme
of connections on a board similar to that shown in Fig. 68.

78. Switchboards for Parallel Bunningr* — When
alternators are operated in parallel, it is necessary to pro-
vide bus-bars and have the different machines arranged so
that they may feed into them. Fig. 70 shows connections
for two three-phase machines arranged for parallel running,
as used by the Westinghouse Company. Main fuses are
here provided between the alternator and main switch, and
these may or may not be placed on the switchboard itself.
The field excitation is carried out in the same way described
in connection with Figs. 68 and 69, about the only difference
being that field plugs Cj d are used instead of field switches.
Three ammeters are provided for each generator, one in
each leg of the three-phase system. In many cases, how-
ever, two ammeters only are used, as shown on the feeder
circuit. T and V are the potential transformers that furnish
current to the voltmeters F, V* and also to the synchronizing
lamps /, V. The voltmeter is also made to serve as a ground
detector by using the plug switches R, R* and ground
keys >&, ^^ The synchronizing lamps are connected to the
transformers by inserting plugs /,^^

79. Usually when a number of alternators are oper-
ated in parallel, it is advisable to have their exciters
arranged so that they may be operated in parallel also. If

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SWITCHBOARDS AND

§25

one exciter breaks down, the others may then sapply the
alternator that would ordinarily be supplied by the disabled

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machine. Again, in large plants, it is quite customary to
supply all the alternators with their field current from one

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§25 SWITCHBOARD APPLIANCES 81

or two large exciters that feed into a pair of exciter bus-
bars, from which the several alternators are supplied.

80. General Arrangrement of Hlgli - Pressure
Switcliboards. — In low-pressure work, the switchboard
consists of a group of slate or marble panels on which the
switches, bus-bars, instruments, and all devices necessary
for the control of the station output are placed. Such crowd-
ing of the parts is dangerous on a high-pressure board, and
the tendency in large stations is to separate the high-pressure
switches and bus-bars so that a short circuit on one part will
not spread to others and' result in a serious interruption of the
service. The switchboard panels in this case carry only the
instruments and small switches necessary for controlling the
main switches that are usually operated either by compressed
air, electric motors, or electromagnets. No parts carrying
high pressure are exposed on the surface of the board, thus
insuring safety to the attendant; a switchboard arranged on
this plan occupies a large amount of space. Fig. 71 shows
a cross-section of the switchboard in the Waterside station
of the New York Edison Company. This board controls the
output of 16 generators, each having a capacity of 4,500 kilo-
watts at 6,600 volts. The board is a good example of a
number that have been installed in modern stations deliver-
ing a large output at high pressure, and brings out the method
of separating the various parts. The main cables from the
generator first pass through the generator oil switch A, and
from there they lead to the two selector oil switches B. The
object of these switches is to allow the generator to be con-
nected to either of the sets of bus-bars C, Z>. There are,
therefore, two oil switches in series between any generator
and the bus-bars into which it is feeding, so that if one
switch fails to operate at any time, the generator can be cut
off by means of the other. From the bus-bars, the current
passes to a non-automatic oil switch E, and then through an
automatic oil switch /% from whence it passes out on the
feeder G. E' and F' are a similar pair of switches for
another feeder. Hy H^ are knife-blade switches that allow

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Pzo. 71

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§26 SWITCHBOARD APPLIANCES 83

any feeder to be connected to either pair of bus-bars. These
switches are never opened while the current is on; other
knife-blade switches K, K' allow switches ^ to be discon-
nected from the bus-bars. The potential transformers used
for supplying: current to the voltmeter, wattmeters, or other
instruments are shown at Z., and the current transformers
are shown at M, It will be noted that all the transformers,
bus-bars, knife switches, and working parts of the oil
switches are separated from each other by brick partitions,
and the various parts are so widely separated that there is
little danger of fire communicating from one to the other.

The instruments connected with the control of the feeders
are mounted in the upper gallery at N, there being a panel
for each feeder. On these panels are mounted the feeder
ammeters, indicating wattmeter, power factor indicator, pilot
switches for controlling the feeder oil switches, and all other
devices connected with the control and measurement of the
outgoing ciuTent.

81. The apparatus for the control of each generator is
mounted on a pedestal at (9, there being a pedestal for each
generator. This pedestal has mounted on it the rheostat
dial switch for adjusting the field excitation of the alternator,
the resistance controlled by this switch being mounted at P
in the gallery below. In addition to this, each pedestal
is provided with a field switch for cutting off the exciting
current, a switch for controlling the engine speed when
synchronizing, synchronizing plug, and pilot switches for
controlling the main generator switches A and the selector
switches B, The ammeters, voltmeters, and other instru-
ments connected with the generators are mounted at ^ on a
small panel immediately above the generator pedestal. By
mounting the generator controlling apparatus on separate
pedestals instead of side by side on panels, the connections
are kept separated to better advantage, and the devices are
also separated, so that there is less danger of throwing
the wrong switches.

The current for exciting the fields of the generators is

45—27

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84 SWITCHBOARDS AND §25

supplied from motor-generator sets S, each consisting of an
alternating-current motor coupled to a direct-current gen-
erator. The apparatus for starting and controlling each of
these sets is mounted on a pedestal T, and the instruments
connected therewith are mounted on panels u directly above
the pedestal. ^ is a low-pressure, direct-current switchboard
from which the exciter current is supplied.

From the above it will be seen that a high-pressure switch-
board for a large station involves a wide variety of apparatus
and occupies a large amount of space. The switchboard used
in the large station of the Manhattan Elevated Railway, New
York, is similar in its general design and handles current at
11,000 volts. In this station the operating board is equipped
with small strips of brass that represent the main bus-bars,
and the handles of the switches are so arranged that when
moved, they apparently close or open the diagrammatic circuit
on the controlling board. Signal lamps are also arranged to
show whether a switch is on or oflE, the whole object being to
arrange the controlling board so that the attendant will see
just what connections exist between generators and bus-bars,
and also what the result will be if certain switches are oper-
ated. The object in arranging the controlling board in this
diagrammatic fashion is to lessen the danger of confusion
when connections have to be rapidly changed — a feature of
special importance where large generating units are involved.

82. Fig. 72 shows a switchboard installation for a high-
tension station of comparatively small output. This view
shows the arrangement of one of the feeder panels. The
lever /, for operating the feeder switch, is placed on
the panel fi that rests on the floor of the lower switch-
board gallery. The levers operate the oil switches A, A by
means of the rods and bell-crank levers, shown in the figure.
One of these rods b is of wood, so that the operating handle
is effectually insulated from the switch. The bus-bars B are
provided in duplicate and consist of copper rods well insu-
lated with oiled tape. They pass through hard-rubber
insulators that are supported by fiber pieces attached to th^

Digitized by VjOOQIC

§25

SWITCHBOARD APPLIANCES

angle-iron framework. Each feeder is provided with a cur-
rent transformer /, none of the indicating instruments being
connected directly to the high-tension lines. Each feeder is
also provided with high-tension enclosed fuses C.

83. Fig. 73 shows the general scheme of connections for
two of the generators and one of the feeders. This layout
may be taken as an example where the generator supplies
current at high pressure to the lines without the intervention

Iff

5 yLL=r j s

w r

¥ h

Fig. 72

of Step-up transformers. Each generator is provided with
an ammeter Z>, supplied from a current transformer /, and
a voltmeter supplied from a potential transformer /,. A
second ammeter C is also connected in the field exciting
circuit, so that the field current may be read at all times.
The current transformer supplies the current coils of the
indicating wattmeter A and the recording wattmeter E.
A indicates the watts delivered by the alternator, and E

Digitized by VjOOQIC

86 SWITCHBOARDS AND §25

records the watt-hours or kilowatt-hours. The mdicating
wattmeter indicates the load on each machine, so that the
attendant can see at a glance whether or not each machine
is taking its share of the load and can adjust the governor on
the engine or waterwheel accordingly. The switch^ is for
connecting the alternator field to the exciter bus-bars, and it
is provided with two long clips between which a resistance A
is connected, so that when the switch is opened this resist-
ance is connected across the field terminals, thus taking up
the discharge from the field and avoiding the danger of
puncturing the field insulation. The construction of this
switch is indicated in the small detail sketch (a). The long
clips are formed so that when the switch is completely
closed, the blades connect the lower and upper clips, but do
not make contact with the middle clips. The synchronizing
plugs are shown at e, e; and /, / are the synchronizing lamps.
Each feeder running out from the station is provided with an
oil switch, fuses, and two feeder ammeters. Sometimes three
ammeters are used on the outgoing lines, as an ammeter on
each line is often of service in indicating the condition of the
line and also in showing whether the load is balanced or not
In some cases the fuses are replaced by automatic circuit-
breakers, while in others the switch is provided with an
automatic tripping device, so that the switch will open the
circuit in case there is an overload or short circuit on the
line. Current transformers J^ are connected in the bus-bars
between the alternators and the feeders in order to supply
total output ammeters.

84. Example of Double-Current Generator Instal-
lation.— Fig. 74 shows a simplified diagram of connections
for two double-current generators feeding into a three-wire,
direct-current system for supplying near-by points and fur-
nishing alternating current, through step-up transformers, to
high-tension feeders running to outlying points. All auxiliary
apparatus, such as ammeters, voltmeters, etc., is omitted in
order to bring out the main connections more prominently.
The method of operation shown in Fig. 74 is used by the

Digitized by VjOOQIC

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Digitized by VjOOQIC

88 SWITCHBOARDS AND §25

Chicago Edison Company. Two double-current generators
are direct driven from a single steam engine and direct cur-
rent at about 125 volts is supplied from the commutators and
three-phase alternating current at from 75 to 80 volts from
the collector rings 1, 2, 5. The commutators are connected
in series and are attached to the neutral bus-bar. The shunt
fields of the generators are arranged for excitation from the
direct-current bus-bars, and the -f and — brushes of the pair
of generators are connected to the + and — bus-bars of the
three-wire system. In order to permit independent control
of the alternating voltage, potential regulators are inserted,
as shown. These regulators are of the induction type
described later in connection with the use of rotary con-
verters. After passing through a low-tension switch, the
alternating current is led to the primaries of three step-up
transformers A, B, C that raise the pressure from 80 volts to
4,500 volts. Each transformer is provided with two primary
coils that are connected to two corresponding phases of the
generators, as indicated by the numbers on the terminals of
the primary coils. Each primary is provided with low-tension
fuses. The two secondaries of each transformer are con-
nected in parallel, and the three groups are A connected to
the high-tension bus-bars. The alternating-current sidles of
the two double-current generators are therefore connected in
parallel through the step-up transformers and feed into com-
mon high-tension bus-bars from which alternating current
at high pressure is supplied to feeders running to distant
centers of distribution. It is thus seen that by using double-
curreijt machines, a variety of service can be supplied from
a single generating outfit and the generators kept loaded to
best advantage.

85. The foregoing will give the student a general idea
as to the arrangement of switchboards and the apparatus
used in connection with them. The variety of apparatus
used in switchboard work is so great that it is impossible
to treat all types. Many stations have now become so large
that it has been found necessary to make the switchboard

Digitized by VjOOQIC

g25 SWITCHBOARD APPLIANCES 89

proper simply a place for grouping the small auxiliary
devices needed to operate the main devices. It is now
common to find field rheostats, field switches, main switches,
etc. operated electrically or pneumatically from a distant
point, and this method of operation has naturally introduced
a large number of new switchboard appliances. Generally
speaking, the tendency is to carry on this remote control by
means of electricity rather than compressed air, as the
electric current has proved just as reliable and is easier
to apply. In some cases small electric motoris are used for
operating switches, rheostats, or other devices, especially
where a rotary motion is required. In other cases a solenoid
or electromagnet is simpler and more easily applied.

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POWER TRANSFORMATION AND
MEASUREMENT

TRANSFORMERS AND TRANSFORMER
CONNECTIONS

1. Tpansformers vary somewhat as to their construc-
tion, but all have the three essential parts, i. e., the primary
and secondary coils or groups of coils and the iron core that

Suspfi

Fig. 1

serves to carry the magnetic flux through the coils. Their
construction also depends to some extent on whether they
are to be used outdoors or indoors. Fig. 1 shows a typical
transformer for outdoor use mounted on a pole in the

For notice of copyrt'zht, see Page immediately following the title pagt
§26

Digitized by VjOOQIC

2 POWER TRANSFORMATION §26

usual manner. Where transformers are large, say above
25 or 80 kilowatts capacity, it is not advisable to mount
them on poles if it is possible to avoid it.

2. Primary Fuses. — Transformers are operated on
constant-potential circuits almost exclusively; hence, if a
short circuit occurs on either primary or secondary, there
will be a heavy rush of current, which will do damage unless
the transformer is instantly disconnected from the circuit.
This is accomplished by inserting fuses in the primary

between the transformer
and the line. The fuses
also protect the trans-
former against over-
loads. Fuses should be
placed in each side of
the primary, as indi-
cated at b, b. Fig. 1, and
should be so mounted as
to be easily replaced by
the lineman. Primary
fuse blocks are made
so that the fuse holder
may be entirely discon-
p^^ nected from the primary

mains when the fuse is
being renewed; in other words, the fuse block serves the
purpose of a switch as well as a fuse holder. In some cases
the blocks are double-pole, but when the primary pressure is
high, it is better to use two single-pole fuse blocks. Double-
pole blocks are not recommended for transformers of greater
capacity than 2,500 watts.

Fig. 2 (^) shows a General Electric double-pole primary
switch and fuse block, with one fuse holder (3) removed for
replacing a fuse. The fuse lies in a deep slot e in the porce-
lain holder {b), and is fastened to the clips d^d. When the
holder is in place, the clips engage with the terminals /, /,
thus completing the connection to the transformer primary.

Digitized by VjOOQIC

§26 • AND MEASUREMENT 3

When a fuse is to be renewed, the porcelain base is pulled
out and the lineman can replace the fuse without danger.

Fig. 3 shows a single-pole block made by the Stanley
Company. In this case, the lid of the iron box is placed at
the bottom and the fuse holder A is pulled out, thus breaking
connection with the terminals /, /. The fuse ^ runs through

(b)

Pio. 8

a block of wood h, thus confining the arc and preventing it
from arcing and burning the terminals /, /.

Where large transformers are operated in substations,
automatic switches or circuit-breakers are used instead of
fuses to disconnect the transformer from the line in case
of a short circuit or overload.

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

TRANSFORMERS ON SINGIiE-PHASE CIRCUITS
3. Transformers in Parallel. — Transformers may be
connected in parallel so as to feed a single circuit, as shown
in Fig. 4, but care must be taken when making the connec-
tions. Suppose that the
two transformers are of
the same type, so that
they will both be wound
alike. The primary ter-
minals Px and /\ must
be connected to one of
the mains, and P^ and
A to the other main;
the secondary terminals
a and c will then have
the same polarity at the
same instant, which is
the result desired. It will be noticed that, from the way in
which the secondaries are connected, they oppose each other,
and that little or no current will flow tmtil the outside circuit
is connected. In practice, it will be found that a current will
flow between the trans-
formers, but it will not
be large. Suppose, how-
ever, that the secondary
terminals are connected
as shown in Fig. 5; the
coils are now in series
sothattheE.M.F.'sact
together to set up a cur-
rent through the coils,
thus resulting in a short
circuit. In connect-
ing up the secondaries,
before making the final connections it is always well to make
sure that the proper secondary terminals are being connected
together. This can be found out by connecting two of them

Pio. 5

Digitized by VjOOQIC

§26

AND MEASUREMENT

together and then connecting the other two through a piece
of small fuse wire or fine copper wire. If the fuse blows, it
shows that the connections should be reversed. It is often
more convenient to reverse the primary terminals than the
secondary, especially if the latter have been joined up per-
manently. Reversing the primary has, of course, the same
effect as reversing the secondary, and it is usually easier to
carry out, because the primary connections are lighter and
easier to handle.

4. Generally speaking, it is not advisable to operate
several transformers in parallel, or banked^ as it is some-
times termed. This is especially true if the transformers are

/hifpary Ma//ts

II 00 DD Q

Secom/ary

Mo/ns

\hM\k\

Pio. 6

small and scattered, as on many lighting systems, although
it was occasionally done some years ago, when transformers
were not made in large sizes. Suppose that a number of
transformers are operating in parallel, as shown in Fig. 6.
If they do not all have the same voltage regulation, the load
may divide unequally between them and one or more of
them take more than its share. The result is that the fuses
of the heavily loaded transformer blow, and a heavier load
is thrown on the remaining transformers, thus blowing their
fuses. Of course, if the transformers are all of the same
size and of similar design, such trouble is not very likely
to happen; but it is better, if possible, to have each trans-
former supply its own share of the load, and if more capacity

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

is needed, to use one large transformer rather than a number
of small ones.

5. Transformers are very often wound with their pri-
maries and secondaries in two sections, so that they can be
connected in series for high voltage and in parallel for low
voltage. For example, in Fig. 7 the transformer is woimd
with two primary coils P, Px, each designed for 1,000 volts
and two secondary coils each wound for 50 volts. By con-
necting the coils Py P^ in series, the transformer may be
operated on 2,000-volt mains, and if the secondaries are also
connected in series, it will supply current to 100-volt second-
ary mains. If the two primaries P, P^ are connected in

^/ooovC^

2000V'

^000^

y/oo0¥^

— Cffre

fOOOibft Primary Matns

\i000^

— Core

tan:

-/ooy-

FiG. 7

Fzo. 8

parallel, as shown in Fig. 8, they may be operated on
1,000- volt mains, and if the secondaries are connected in
series, they will supply current at 100 volts. If desired, the
secondaries could be connected in parallel to supply cur-
rent at 50 volts, but the 50-volt secondary circuit has
practically gone out of use. A pressure of 50 volts was, at
one time, used for incandescent lamps operated from trans-
formers, but has given place to 100 to 110 volts, because the
latter pressure requires less copper and it is now possible to
obtain 100- to 110- volt lamps that operate fully as satis-
factory as those made for 50 volts. Transformers are now

Digitized by VjOOQIC

§26

AND MEASUREMENT

frequently wound so that they can be connected for either
104 or 208 volts on the secondary.

6. In many places, plants that were origfinally installed
to operate at 1,000 volts primary pressure have been changed
to 2,000 volts, in order to allow a larger load to be carried
without increasing the size of the line wires. In such cases
it has been common practice to connect old 1,000- volt trans-
formers in pairs, as shown in Fig. 9.

7. Transformers on tlie Tliree-Wlre System. — ^The
general tendency is to use a few large transformers for
supplying a given district rather than a number of small
ones. Small trans-
formers are wasteful
of power, and though
each in itself may
not represent a very
large loss, yet when
a large number are
connected the total
amount of energy
that might be saved
during a year by
using a few large
transformers may be
surprisingly large.
Of course, in most cases where the customers are scattered it
is impossible to avoid using a number of small transformers,
but in business districts it is generally easy to use a few
large transformers of high efficiency. These are frequently
connected in pairs so as to feed into three-wire secondary
mains mym^m^ as shown in Fig. 10. The primaries are
connected directly across the line in parallel, and the second-
aries are connected in series with the neutral wire connected
between them at the point o. Care must be taken in con-
necting the secondaries to see that the terminals a, b are
of opposite sign. If they are correctly connected, a pair
of lamps /, / connected in series acro$s the outside lines

Pio.9

Digitized by VjOOQIC

POWER TRANSFORMATION

S96

should bum at full brightness. If they are wrongly con-
nected, the lamps will not light at all, showing that terminals
Uy b are of the same polarity and that Cy d are also the same,
the secondaries being connected so that the two outside
mains are of the same polarity with a common return wire
in the middle. If two transformers are of the same style
and make, the terminals of corresponding polarity will usually
be brought out of the case in the same way. For example,
in Fig. 4, terminals a^ c would be of the same polarity at
the same instant. It is always best, however, to test out
the connections before connecting permanently, and this is

/h'ma/y Mains

\ aOOOV H[ \^— 2000 V 4

T/inee Wire Seconchry [ A4ains <{) <{><{><{><{>[

^oo->

Pio. 10

f I

especially necessary in case two transformers of difiEerent
make or type are being dealt with.

8. Core-Type Transformers on Tlipee-Wipe System.

When ordinary transformers of the core type are used to
supply current to a three-wire secondary system^ as shown
in Fig. 11 (a), the voltage on the two sides of the circuit
may become greatly unbalanced if the load is not equally
divided. For example, in Fig. 11 {a) take the extreme case
where the side a is not loaded at all. Secondary coil s will
have no current and will therefore set up no counter mag-
netization, whereas coil s! will have a current due to the load
on side b. Thus the magnetic flux in the two sides of the

Digitized by VjOOQIC

§26 AND MEASUREMENT 9

core becomes unequal, as roughly indicated by the dotted
lines, and the secondary E. M. F. is considerably higfher on
the side a than on the loaded side b. In order to overcome
this difficulty, the General Electric Company wind the sec-
ondary in a number of sections ^,5,5,5, Fig. 11 (3), and
cross-connect these coils as indicated. The result is that no
matter how unbalanced the load may be, the magnetizing
effect of the secondary is the same on both cores and the
voltage remains practically the same on both sides.

Pio. U

TRANSFORMERS ON TWO-PHASE CIRCUITS

9. As most two-phase circuits are operated with foiu:
wires, such a system is practically equivalent to two single-
phase circuits. If it is necessary to connect two trans-
formers in parallel, as shown at (a), Fig. 12, their primaries
must be connected to the same phase. If they are connected
to different phases, as indicated by the dotted lines running
to phase 1, a. local current will flow through the secondary
coils, because the secondary currents will not be in phase
and there will be intervals when the E. M. F. of one will
be greater than that of the other. The secondaries may,
however, be connected in series, as shown at (d), forming a

45—28

Digitized by VjOOQIC

^^^

»$

1^ "

Digitized by VjOOQIC

§26 AND MEASUREMENT 11

kind of three-wire system. If the voltage of each secondary
is E^ the voltage between the two outside wires will be
EY. 1.414. This is because the E. M. F.'s in the two coils
are not in phase. This method of connecting transformers,
however, is not to be recommended, as the voltages on the
two sides of the three-wire system are apt to become
unbalanced. If a three-wire system is desired, it is better
to use the connections shown at (^), where both primaries
are connected to the same phase. The E. M. F.*s in the two
secondary coils are, in this case, in phase with each other
and the pressure across the outside wires is twice that of
one secondary coil.

10. In connecting transformers to a two-phase system,
the aim should be to get the load on the two phases as
nearly balanced as possible. Of course, where motors are
operated, both phases are used, and, hence, there is not
much danger of an tmequal division of load. When lamps
are connected, one transformer or set of transformers at
one point on the circuit can usually be balanced against
another group at some other point, so that the load as a
whole will be equally divided. Fig. 13 shows different
methods of connecting transformers on a two-phase system,
using three line wires. In this case the central wire acts as
a common retiurn, and the voltage between the outside wires is
1.414 times that of each phase. The same remarks apply
here as in the previous case, and the three- wire arrangement
shown at (^) is not as generally satisfactory as that shown at
(^). In both cases the primary pressure is shown as 2,000
volts, and transformers with a ratio of 20 to 1 are taken for the
sake of illustration.

TRANSFORMERS ON THREE-PHASE CIRCUITS

11. Until recently it has been customary in America to
use three single-phase transformers for transforming from
one pressure to another on three-phase circuits; the three
transformers may be connected up either Y or A. With
the A arrangement, the power supply will not be entirely

Digitized by VjOOQIC

I .^

Digitized by VjOOQIC

§26 AND MEASUREMENT 18

crippled even if one of the transformers should become
damaged; also transformers wound ^or standard line volt-
ages can be used. In some cases, however, the primaries
are connected across the lines according to the Y scheme, as
shown at {a)y Fig. 14, and since there are two primary coils
in series between any pair of mains, the pressure on any one
primary coil is less than that between the mains. Wheti the
primaries are Y connected, the secondaries are usually Y con-
nected also, as shown at (a). Sometimes, however, the
primaries are Y connected and the secondaries A, as shown
at id). If transformers having a ratio of 20 to 1 were con-
nected in this way, the secondary pressure would not be the

100
primary pressure divided by 20, i. e., 100 volts; but Tif^' ^^

67.7 volts. In order to get 100 volts secondary with this
scheme of connections, the transformers would have to be

20
wound with a ratio of 7~7^ ^^ ^* ^' ®'» H-^S to 1, approxi-
mately. Fig. 14 {c) shows transformers with both primaries
and secondaries A connected. The arrangements shown at
(a) and {c) are the ones commonly used for three-phase
work, as scheme (d) either calls for special windings on
the transformers or else gives rise to odd secondary volt-
ages. If the prin:iaries are to be A connected, each primary
coil must be wound for the full-line voltage. If the pri-
maries are Y connected, each primary coil is wound for the
line pressure divided by 1.732. It is possible to use only
two transformers on a three-phase system, as shown in
Fig. 14 (af ) , but this arrangement is not, on the whole, as desir-
able as the A connections, because if one breaks down the
service is crippled. It is equivalent to the delta arrange-
ment with one side left out. The connections shown in (c)
are used more largely than any of the others.

12. Phase-Chanf^n^ Transformers. — By combining
two E. M. F.'s that differ in phase by 90°, an E. M. F. of any
desired amount and phase relation to the original E. M. F.*s
can be obtained. For example, in Fig. 15 (a), suppose it is
desired to produce an E. M. F. E of the amount represented

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

by the line oc and having the phase relation of oc. This
E. M. F. can be regarded as made up of the two com-
ponents ob and (7 a at right angles to each other; hence, if
two E. M. F/s Ex and E^, having the values represented by
the lines ob and oa, and differing in phase by 90°, are com-
bined, the result will be the required E. M. F. E, In Fig. 16(^) ,
A and B are the primaries of two transformers connected
to a two-phase system. The E. M. F.*s Ex and E^ induced
in their secondaries will therefore differ in phase by 90° and
Ex and E^ can be made any desired value by suitably

\sm£0

ssmw

X Id

'JB2

loom.

Fio. 15

proportioning the windings. If the two secondaries are con-
nected in series, the E. M. F. between the lines will be the
geometric sum of Ex and E^, as shown in {a). For example,
in (b), in passing from line 1 to line 2 we go through each coil
in the same direction; that is, we pass from a to b and from
^ to fl? in the direction indicated by the arrows. We will call
this the positive direction. In (d), in passing from a to ^ we
go through the coil a b in the positive direction, but, with the
connections of the second coil reversed, as shown, we pass
through cd from d to c against the arrow. The line oa (c) is
therefore reversed with regard to its position in (a) and

Digitized by VjOOQIC

§26

AND MEASUREMENT

the E. M. F. E between lines 1 and 2 is now denoted by the
line oc^ which is the same in amount as in {a)y but has a
different phase relation. Fig. 15, therefore,. shows a method
of obtaining a single phase current of any desired amount
or phase relation, from two currents diflEering in phase by 90°.

13. Scott Two-Phase, Tliree-Pliase Transformer.

One of the most common examples of phase transformation
is the changing of two-phase currents to three-phase, or vice
versa, by means of the arrangement devised by Mr. C. F.
Scott. In Fig. 16 (a), A and B are the primary coils of two
transformers connected to a two-phase system. The second-
ary of A, i. e., the coil a r, is provided with a winding such

KsmssmsJ

nsmPi

PlO. 16

that its voltage E will be the required voltage of the three-
phase system. The secondary of B has -x- or .87 times as

many turns as the coil a Cy so that the voltage generated in
it is .87 E. One end of coil de is connected to the middle
point d of coil a c, as shown. With this arrangement of wind-
ings and connections, three currents differing in phase by
120° will be delivered to lines 1,2,3 when the primaries are
supplied with two currents differing in phase by 90°. The
same connections are shown in a simplified form in (^),
the three-phase lines being attached to points i, 2, and 5. The
E. M. F. between 1 and 2 is that generated in the secondary

Digitized by VjOOQIC

16 POWER TRANSFORMATION §26

ac. The E. M. F. between 2 and 3 is the E. M. F. generated
\n he combined with the E. M. F. generated in he. The
E. M. F. between 8 and 1 is that in ^^ combined with that
in ba. It must be remembered that the E. M. F. in he is
at right angles to the E. M. F.'s \nab and be. Coming back
to (a) and noting that the positive direction through the
coils is marked by the arrows we can lay off the line o e
in {e) to represent the E. M. F. between lines 1 and 2.
The E. M. F. between points a and b is marked a — ^ in
(r) and is represented by one-half of oe. Also, the E. M. F.
between b and e would be represented by ^ — r. The — sign
does not here signify subtraction; it simply denotes that the
E. M. F. referred to is taken between the points b and e.
The E. M. F. between lines 2 and 3 is found by adding,
geometrically, the E. M. Y, d^ e X.o c —h. In passing from
line 2 to 5 we pass from ^ to ^ against the arrow, or in other
words the E. M. F. f — ^ is the equal and opposite of ^ — ^
and is represented by ^/ in (^) equal to one-half of oe, but
drawn to the left of o. Coil de is passed through in the
positive direction so that the E. M. F. ^ — ^ will be repre-
sented by the line oh above the horizontal, and the E. M. F.
between lines 2 and 3 will be the resultant of ^/ and oh, or
of. The E. M. F. between lines 3 and 1 is e ^ d combined
with h " a. The E. M. F. between e and d is the equal and
opposite of that between d and e; hence, it is represented by
ok, which is equal and opposite to oh. The E, M, F, b — a
is equal to and in the same direction as ^ — b; hence, it is
represented by o /, and the resultant of ^ / and ^ ^ is o£', which
is the pressure between lines 3 and 1. The three secondary-
line pressures represented by the lines oe, of, and ^^, are
therefore of equal amount and differ from one another in
phase by 120°, as is required for a three-phase system.

For long transmission lines, it is more economical to use
the three-phase than the two-phase system; hence, where
power is generated by two-phase alternators and stepped up
for transmission over long distances, as, for example, at
Niagara, the current is often transformed from two-phase to
three-phase as just explained.

Digitized by VjOOQIC

126

AND MEASUREMENT

14. Capacity of Transformers on Trvo- and Three-
Phase Systems. — When transformers are connected on a
two-phase system each transformer must be of capacity suffi-
cient to carry half the load. If the three-phase system using
three transformers is used, each transformer must be capable
of carrying one-third the load. When the transformers are used
to operate induction motors, a safe plan to follow is to install
1 kilowatt of transformer capacity for every horsepower
delivered by the motor. Thus, a 20-horsepower, two-phase,
induction motor will require two 10-kilowatt transformers;
a 30-horsepower, three-phase motor will require three 10-
kilowatt transformers; and so on. Table I, issued by the
General Electric Company, shows the size and number of
transformers suitable for 60-cycle, three-phase induction
motors.

TABIiK I

CAPACITT OF TRANSFORMERS FOR

THREE-PHASE INDUCTION

MOTORS

Horsepower
of Motor

I
2

5
7i

30
50

Capacity of Transformers
Kilowatts

Two
Transformers

Three
Transformers

1.5

I.O

2.0

1.5

3.0

2.0

4.0

3.0

5.0

4.0

7.5

5.0

10. 0

7.5

15.0

10. 0

25.0

15.0

25.0

Digitized by VjOOQIC

18 POWER TRANSFORMATION §26

SUBSTATION EQUIPMENT

15. General Features. — The high-tension alternating
current, for large transmission systems, is usually delivered
to a number of substations rather than to scattered groups
of transformers, and it is therefore necessary to study the
equipment of these substations. In some cases the power
is delivered from the substation in the shape of alternating
current; in others, it is transformed to direct current and
delivered to the various receiving devices, such as lamps,
motors, etc. Part of the output may be delivered as direct
current and part as alternating, either at the same frequency
as the current generated in the main station or at a different
frequency. It is thus seen that the character of the equip-
ment in a substation may vary greatly, and will depend on
the character of the service. If the power is used for oper-
ating a street railway where direct current at a pressure of
500 to 600 volts is required, the substation must be equipped
with rotary converters for changing the alternating current
to direct. Also, since the alternating current is transmitted
at high pressure, it is necessary to provide transformers to
step-down the incoming line voltage to an amount such
that the converters will give the required direct-current
voltage. The current can also be transformed from alter-
nating to direct by using motorrgenerator sets, i. e., sets
consisting of an alternating-current motor connected to one
or more direct-current generators. Motor generators are
more expensive than rotary converters of equal output,
and are not quite so efficient; hence, the latter, especially
in America, are much more generally used. For some
classes of work, motor generators have advantages, and
their operation on fairly high frequencies, over 60 cycles,
is more satisfactory than that of rotary converters. They
are used considerably on 60-cycle systems where the direct
current is used for lighting work which requires close

Digitized by VjOOQIC

§26 AND MEASUREMENT 19

voltage regulation. In a motor-generator set the two sides
of the system are enlirely separated, and disturbances on
one side are not so liable to affect the other as with
rotary converters. It is often practicable to wind the motor
to take the high-tension line current without the inter-
vention of step-down transformers, but even allowing for
this the motor generator is not as economical, either as
regards first cost or efficiency of operation, as the rotary
converter. By using frequencies from 40 to 25 cycles per
second, little difficulty is found in operating rotary con-
verters; and at these frequencies they are largely used
for the conversion of alternating current to direct current,
or vice versa.

16. In some cases the output of a substation is delivered
wholly as alternating current, and the substation contains
simply the static transformers needed for raising or lower-
ing the pressure, together with the switchboard appliances
used to control the incoming and outgoing current. In sub-
stations where the output is in direct current supplied to
lighting or railway systems, it is common practice to provide
a storage battery in order to equalize the load, the battery
being charged during intervals of light load and discharged
when the heavy load comes on. The use of a number of
substations supplied from one large central station results in
a comparatively constant load on the central station, especially
when storage batteries are used in those substations that are
situated in densely populated districts and are called on for
a very heavy output at certain hours during the day. One
of the chief advantages in supplying the power from a large
central station is the uniformity of load obtained throughout
the day, thus allowing the generating units to be worked at
their best efficiency.

The equipment of a substation may be conveniently con-
sidered under three heads, namely: (a) Apparatus for Con-
trolling the Incoming Current; (d) Apparatus for Transform-
ing the Current; (c) Apparatus for Controlling the Outgoing
Current.

Digitized by VjOOQIC

So POWER TRANSFORMATION §26

APPARATUS FOR CONTROIiliINO THE INCOMING

CURRENT

17. The apparatus for controlling the incoming current
is generally grouped on a regular high-tension switchboard,
and is separated, at least so far as the high-tension parts are
concerned, from the devices controlling the outgoing current.
If lightning arresters are used, they are placed at a point
near where the wires enter the building; very often they are
placed in a separate buildings The arrangement of the con-
trolling devices, of course, differs in different stations, but
the incoming lines should first pass through a circuit-breaker
or main switch so that all current may be cut off from the
station. In many cases oil switches are used, and are so
arranged that they may be either opened by hand or auto-
matically whenever the current exceeds the allowable amoimt.
Arranged in this way, the switches fulfil the requirements of
both a circuit-breaker protecting the apparatus in case of
overload, and a main switch that can be opened by hand
when desired. Switches of the air-break type and those in
which the arc is broken in a confined air space are also made
to operate automatically in case of overload; all of these
types are in common use for protecting the incoming lines.

18. Tlme-tilmlt Relay. — In most substations, espe-
cially in those where rotary converters are operated, it is
not desirable to have the circuit opened every time there is
a momentary overload, because it allows the converters to
fall out of synchronism and it takes some time to get things
tmder way again. Besides, momentary overloads will not,
as a rule, damage anything, while a long continued overload
or short circuit will. For these reasons it is advisable to
equip the circuit-breakers, or automatic switches, on the
incoming lines with a tlme-llmlt relay, which controls
the current in the tripping coils and will not allow the circuit
to be opened until a certain interval of time has elapsed
after the occurrence of the short circuit or overload. If the
overload should pass off diuing this interval, the relay goes

Digitized by VjOOQIC

§26

AND MEASUREMENT

back automatically to its initial position, and the circuit is
not opened. If, however, the overload should continue
beyond the limit for which the relay is set, contact is made
and the tripping coils energized, thus opening the circuit.

Time-limit relays have been made in a variety of forms.
Fig. 17 shows one type intended for two-phase or three-
phase circuits and used on a number of the Niagara lines.
The coils a, a are connected to the secondaries of current
transformers whose primaries are in series with the main
lines. If thfe current in either phase exceeds the allowable
amount, either one or both of the armatures b, b are pulled
down, thus releasing
the clockwork mech-
anism r. If the short
circuit or overload is
not removed within
the time limit for
which the relay is
set, say 3 to 5 sec-
onds, the clockwork
makes a contact that
allows current to flow
through the tripping
coil of the circuit-
breaker and thus
opens the circuit. If
the overload or short
circuit should disappear during the time limit, the armatures
b, b rise, thus preventing the clockwork from making contact.
By equipping the various circuit-breakers on a system with
this attachment, it is possible to set them so that in case a
short circuit or overload occurs on a certain section, the
circuit-breaker nearest that section will go out before those
nearer the station. In other words, the breakers near the
station are set so as to hold on for a longer interval than
the more distant ones, thus preventing a shut-down of the
machinery due to some fault on a distant part of the system.
The time that must elapse before the relay makes contact

Fig. 17

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

can be adjusted by varying the angle made by the vanes d^

Fig. 17. Fig. 18 shows the connections for one type of high-
T9iin0. tension circuit-breaker

operated by a time -limit
relay. Current is supplied
to the coils of the relay by
the secondaries of the cur-
rent transformers Ay A'.
The incoming lines are
attached to sfaids a, a of
the circuit-breakers, and
the main current crosses
over to studs b,b by way
of the laminated contacts
c, f, which are forced up
against the studs when the
breaker is set. Each pair
of contact studs a b \s
shimted by a long enclosed
fuse mounted in holders so
that it can be quickly re-
placed by a new one in
case it blows. When the
breaker opens, thus with-
drawing c from a and ^, the
main current flows momen-
tarily through the fuse and
the circuit is, therefore,
finally opened by the fuse,
which is capable of taking
care of the arc. If the cur-
rent becomes excessive
and holds on beyond the
time limit for which the
relay is set, contact d
^^°-^ touches e, thus allowing

the cells / to send a current through the tripping coils of

the breaker.

^ '"^ '<

•Switeh.

0 Q II

Digitized by VjOOQIC

§26 AND MEASUREMENT 23

19. Westiniirliouse Tlme-Iilmlt Relay. — Fig. 19 shows
a relay made by the Westinghouse Company. In this case
the time-limit feature is regulated by means of a dashpot.
A solenoid a is connected to the secondary of the current
transformer, and the movable core b rests on a lever c pivoted
at d. To the end of c is attached the piston rod e, which
carries the piston of the dashpot /. The lever f, counter-
balanced by the weight g, is normally held in the position
shown in the figure, by the weight of core b resting on it.
The arm A, also pivoted at d, carries the contact springs
ky I and its position can be adjusted, up or down, by an
adjusting screw on the

cover of the instrument.
Lever c carries a contact
piece m that connects
ky I if lever c rises far
enough. When the cur- ,
rent in a exceeds the
allowable amoimt, core]
b is lifted, thus allow-
ing the counterweight g
to raise lever c. The
movement of c is con-
trolled by the dashpot /
and the time during
which the overload may
exist before the circuit pio. 19

is opened is determined by the position of arm h. When
lever c has moved high enough to make contact between
k and /, the circuit-breaker is tripped and the main circuit
opened. Should the overload pass off before the time limit
is reached, b drops back and lever c is forced down before it
has had time to make contact between k and /.

20. Reverse-Current Relay. — In a large distributing
system where a number of substations are connected to the
main station, and to each other, by a network of cables,
it is necessary to provide some means for preventing current

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

from flowing back toward a defective part and thereby main-
taining a short circuit. This point will be understood more
clearly by referring to Fig. 20, where A is the main station
from which current is supplied to the substation B. Usually
a number of cables in parallel are run between the main
station and the substations in order to allow the use of cables
of reasonable dimensions, and also to provide for tminter-
rupted service in case one or more cables should break down.
Suppose that c and d represent two three-wire cables, supply-
ing the substation B with three-phase current. When both
are in use, the ends at the substation and at the main station
are connected to common bus-bars. Suppose that a short
circuit occurs at / on cable c. The rush of current through

Pio.20

the fault will, of course, open the circuit-breaker on cable c
at the main station, but since d and c are connected together by
the substation bus-bars, there is nothing to prevent a heavy
current from flowing out over d and back through c to the
fault /, thereby causing the circuit-breakers of cable d to
open and completely shut off the power from the substation.
In order to prevent this, reverse-current relays are
installed at the end of the feeders, and their duty is to
trip the circuit-breakers the instant the flow of energy
through any of the cables reverses. Of course, where a
substation is supplied by a single set of feeders and fur-
nishes current to a secondary system which is not capable

Digitized by VjOOQIC

i26

AND MEASUREMENT

of feeding current back to the line, reverse current relays
are not needed.

Fig. 21 shows an arrangement of reverse-current relays
used on the Niagara system, and also in a number of other
installations. Ay A are the
circuit-breakers, and B^ B
the reverse-current relays.
These relays are similar
in construction to small
direct-current motors
having laminated fields.
The field windings are
excited by current from
the secondaries of two
potential transformers /, /^
and the armatures are sup-
plied with current from the
current transformers c^ c^.
The armatures are not
allowed to turn, since their
motion is limited by an
arm playing between two
stops as shown. When the
current is flowing in its
normal direction from the
cables to the bus-bars, the
arm of the relay bears
against the lower stop,
which is not connected
electrically to any other
part. If, however, the flow
of energy is from the bus-
bars to the cables, the flow
of current at each instant
in the armature is reversed with respect to that in the fields,
and the armature at once swings around in the opposite
direction until the arm touches the upper stop, thus closing
the battery circuit and tripping the circuit-breaker.

45—29

Fio. a

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26 POWER TRANSFORMATION §26

This feeding-back action can also occur, if reverse-current
circuit-breakers are not used, where a substation supplied
through even a single set of feed-wires runs rotary con-
verters which, on their direct-current side, are in parallel
with storage batteries. If a short circuit occurs on the
cable and it is cut off from the main generating station,
the converters can still operate with direct -current fur-
nished by the battery. They thus nm inverted, taking the
direct current from the batteries, converting it into alter-
nating current, and feeding back to the line through the
transformers. The current thus fed back to the fault in
the cable will be very large, and may cause injury to the
apparatus if means are not taken to prevent it by means
of reverse-current circuit-breakers!

APPARATUS FOR TRANSFORMING THE CURRENT

21. If the current supplied from the substation to the
consumers is utilized as alternating current, the substation
is equipped with step-down transformers that supply alter-
nating current directly to the secondary network. If the
current is utilized as direct current, it is necessary to install
rotary converters or motor generators in addition to the
step-down transformers.

22. Substation Transformers. — Transformers used
in substations do not differ materially from ordinary trans-
formers except as regards their size and the methods
used to secure cool running. They are usually of very
large output as compared with those used for ordinary
local lighting and power distribution. Their efficiency
is very high, but on accoimt of the comparatively small
radiating surface that they present to the air, it is neces-
sary to provide special means for getting rid of the heat,
either by means of an air blast or by water that circulates
through a coil of pipe placed in the upper part of the
transformer case. With the latter method, the transformer
case is filled with oil, and ^s the heated pil ris^s to the

Digitized by VjOOQIC

§26 AND MEASUREMENT 27

upper part of the case it is there cooled by the water in
the pipes, and descends to the lower part, thus keeping up
a continuous oil circulation that carries the heat away from
the coils and core.

Fig. 22 shows a Westinghouse 2,250-kilowatt substation
transformer; (a) shows the coils and core assembled before
being placed in the case. The core laminations a, a are
built with openings d, b at intervals so that the oil can
circulate through the core and conduct the heat from the
internal parts. The primary and secondary coils are each
wound in several sections in the form of large flat coils,
which are then sandwiched together, making a construc-
tion that reduces magnetic leakage, and at the same time
cuts down the voltage generated in any section of the
winding. The ends of the coils project beyond the lamina-
tions at the top and bottom as shown at c^ and the terminals
of the coils lead to a terminal board mounted on top. The
transformer is placed in a cylindrical tank made of riveted
boiler plate. Fig. 22 (^), and is completely submerged in
oil. Four coils of pipe placed in the upper part of the
tank are connected in parallel by pipes a, a attached to
common inlets and outlets. Each coil is provided with
a valve, so that in case it becomes defective, it can readily
be cut out without disturbing the flow of water through
the others. This transformer, being of very large output,
has an efficiency of 98.63 per cent, at full load, 98.2 per cent,
at half load, 97.2 per cent, at quarter load, and 98.5 per
cent, at one-half overload.

Fig. 23 shows a sectional view of an air-blast transformer
of the General Electric type. The construction of the
coils A, A and core B, B is such that air spaces are left
between the parts, and the transformer is moimted over an
air chamber in which about i ounce air pressure is main-
tained by motor-driven fans. The air passes through the
openings in the core, between the coils, and out at the top
and sides; suitable dampers are provided by means of which
the flow can be regulated. This makes an efficient and
cleanly method of cooling large transformers.

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

Digitized by VjOOQIC

§26 AND MEASUREMENT * 29

Fig. 24 shows a group of nine air-blast transformers of
150 kilowatts each. A motor-driven fan is mounted at each
end of the chamber and either fan has sufficient capacity to
keep the transformers cool, thus providing a reserve blowing
outfit in case one breaks down. The power required to
operate the fans does not usually exceed one-tenth of 1 per
cent, of the transformer output.

23. Polyphase Transformers. — In Europe, two-phase
and three-phase transformers have been quite commonly
used, and three-phase
substation transformers
are now manufactured
in America. By using
polyphase transformers,
a saving in material is
efiEected, thus reducing
the cost per kilowatt.
Also, a considerable
saving in space is gained
because a polyphase
transformer, of given
output, takes up less
room than an equivalent
output in single-phase
transformers. This is i
an important considera-
tion in stations located
in large cities. On the F10.23

other hand, the use of single-phase transformers is some-
what safer, because if a breakdown occurs it is liable to
damage but one of the transformers.

Fig. 25 shows the general arrangement of a three-phase
core-type transformer. The primary and secondary coils,
which are wound on the cores A, By C, may be connected
Y or A. The magnetic flux in the core follows the same
changes as the currents. Each core acts alternately as the
return path for the flux in the other two cores, just as each

Digitized by VjOOQIC

30 POWER TRANSFORMATION §26

line wire acts alternately as the common return for the other
two in a three-phase line. The iron in the core is thus worked

Pio.24

to better advantage than when three separate single-phase
transformers are employed. A two-phase transformer can

be made by winding
coils on cores A and C
and leaving core B
without' coils; B will
then act as the return
path for the fluxes set
up by the coils or^.A
and C. Since these
two fluxes will difl^er in
phase by 90^, the re-
sultant flux in B will be
^^^•^ V2 times the flux in A

or C; hence, for a two-phase transformer, the central core B
will have a cross-section V2 times that of ^ or C instead of
being equal as shown for the three-phase transformer.

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§26 AND MEASUREMENT 31

BOTART COXV£RTBBS

24. The main features of rotary converters were
described in connection with alternating-current apparatus.
The types generally used are the two-phase or quarter-phase,
three-phase, and six-phase; in America, the three-phase
converter is used more largely than either of the others.
Each converter is provided with its transformer or set of
transformers in case it is necessary to step-down the line
voltage. In some stations, notably in railway power plants,
the alternating current is generated at low pressure when the

Hfgh Tmnstort Bus-Sars \ i

— r— y i"i —

Pig. 26

greater part of the power is used near the station. In such
plants, the near-by portions of the system are supplied with
direct current from rotary converters placed in the main
station and supplied with current directly from the alternators
without the intervention of step-down transformers. If a
very large percentage of the power was used as direct
current for near-by points it would probably be cheaper to
install double - current generators and dispense with the
converters. In the majority of cases, however, where

Digitized by VjOOQIC

32 POWER TRANSFORMATION §26

converters are used it is necessary to use transformers to
supply a suitable voltage.

25, Connections for Six-Phase Rotary Converters.

It has been shown that the output of a rotary converter is
increased by increasing the number of phases, and six-
phase converters are used to a considerable extent, especially
where the machines are of large output. Six phases are
easily obtained from three by providing each of the three
transformers with two secondary coils, as shown in Fig. 26.
Coils i, 5, and 5 are connected A, as also are 2, 4, and 6,
one group being reversed as regards the other, thus giving
the double-delta arrangement indicated in Fig. 27. The
collector rings are attached to the points a, b, c, etc., thus
supplying the converter with six currents differing in phase-
by 60°. The use of six phases introduces
some additional complication in the connec-
tions between the transformer secondaries
and the converter, and also requires six
collector rings, but this extra complication
is more than offset by the increased output
of the converters. Sometimes switches are
• inserted between the transformer secondaries

and the converter, but more often the switching is done on
the primary side because the secondary current is usually
large and the switching devices correspondingly heavy.

26. Voltage Kegrulation of Rotary Converters.

Usually it is necessary to arrange converters so that their
direct-current voltage can be increased with increase of load
so as to keep the voltage constant at distant points on the
system. It was pointed out in connection with the theory
of rotary converters, that the voltage of the direct-current
side could be raised or lowered within certain limits by
changing the field excitation of the converter. The change
in field excitation with increase in load is usually obtained
by providing the machine with a compound field winding
similar to that on a compound-wound, direct-current dynamo.
If the load were not of a suddenly fluctuating character, the

Digitized by VjOOQIC

§26

AND MEASUREMENT

necessary field regulation could be obtained by adjusting: the
rheostat in the shunt-field circuit, and a series-field winding
would not be needed.

In order to admit of voltage regulation by varying the
field strength of the converter, it is necessary to have a cer-
tain amount of reactance on the alternating-current side;
this can be provided by inserting reactance coils between
the transformers and the collector rings, as shown in Fig. 28.
A, B, and C are the
step-down transform-
ers, and Z7 is a lami-
nated core on which
the three reactance
coils are wound.

Another method of
regulating the volt-
age of a converter is
to provide the trans-
former secondaries
with a number of taps
connected to a multi-
point switch, thus
allowing the number
of secondary turns
to be varied. This
method does not ad-
mit of as gradual a .
variation in voltage
as some others, but
it is simple and well
adapted to cases where a
regulation is desired.

A third method of regulation is to insert a potential reg-
ulator between the transformer secondaries and the collector
rings. These regulators are made in a variety of forms, but
they are nearly always some special type of transformer; the
general features of this method of regulation will be under-
stood by referring to Fig. 29. The secondary coils s^ s, s of

Pio.28

considerable range in voltage

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POWER TRANSFORMATION

§28

0(XXXfflXKX».ri.OQQQQQOQQQttrUMO^

the regfulator are connected in series with the leads running^
between the transformers and the converter; the primaries
p.p^p are connected across the three phases as shown.
Since the secondary coils are in series, with the mains, it is
evident that their E. M. F.'s will be added to or subtracted
from those of the main transformers. If provision is made
for varying the value of the E. M. F.'s generated in ^, s, s, or
for changing their phase relation with respect to the E. M. F.'s
of the main transformers, the E. M. F.'s applied to the con-
verter can be raised or lowered by an amount equal to the
pressure generated in s. In some regulators, the effective

E. M. F. of the series-coils
is varied by cutting turns
in or out, as, for example,
in the Stillwell regulator.
Provision is also made for
reversing the E. M. F. of
the coil with respect to the
circuit, so that the main
E. M. F. can be raised or
lowered. Another scheme
is to arrange the magnetic
circuit or the secondary
coil so that by moving
the coil or a portion of
the core, the amount of
magnetic flux passing
through the secondary can be varied, thus changing the
value of the induced E. M. F.

27. Fig. 30 (a) shows the general appearance of a three-
phase induction potential regulator made by the Greneral
Electric Company and intended for regulating the voltage of
a rotary converter. The stationary part of this regulator.
Fig. 30 (d), consists of a laminated structure a, a with
inwardly projecting teeth exactly similar to the field of an
induction motor. This is provided with distributed bar
windings d, d, which are connected in series with the mains

Pio. 29

Digitized by VjOOQIC

§26 AND MEASUREMENT 35

running: to the converter. The primary consists of a lamina-
ted core c^ c similar to the armature core of an induction
motor; this is mounted on a vertical shaft s so that the core
can be turned through a limited range by means of the hand
wheel h^ which operates a worm engaging with a segmental
gear attached to s. The primary is provided with three wind-
ings distributed in the slots, and connected across the phases
as described in connection with Fig. 29. In this type of regu-
lator, the field set up induces an E. M. F. of constant amount in
each secondary winding. The adjustment of the amount of
"boost*' is efiEected by varying the phase relation of the sec-
ondary E. M. F. to that in the primary. For example, if the
secondary induced E. M. F. and the primary E. M. F. are in
phase, i.e., with the north and south poles of the primary and
secondary windings facing each other, the maximum amount
of increase in voltage will be obtained. With the secondary
E. M. F. exactly opposite in phase to the primary, the E. M. F.
will be lowered by an amount equal to the induced E. M. F.
For intermediate positions of the primary, intermediate phase
relations are obtained and the E. M. F. will be raised or
lowered by an amount corresponding to the value of the com-
ponent of the secondary E. M. F. that is in phase with or in
opposition to the line E. M. F. With a regulator wound for
four poles, a movement of 90° will give the total range of
voltage, and as the movement is not large the current can
be conducted into the primary by means of flexible cables.
These regulators are also arranged for operation by means
of a small motor, thus allowing them to be placed at some
distance from the switchboard.

28.- Methods of Starting: Rotary Converters. — In

cases where direct current is available, rotary converters
are usually started by driving them up to synchronism as
direct-current motors. In many substations, storage batteries
furnish a source of direct current that is available at all
times for starting purposes. Of course, when one converter
has been started it can be used as a source of direct current
for starting others. In some cases, where a storage battery is

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36 POWER TRANSFORMATION §26

not available, direct
current is obtained
from a small motor-
generator set con-
sisting of an induc-
tion motor coupled
to a direct-current
dynamo. One advan-
tage in starting from
the direct-current
side is that the direct
current furnished by
the converter is al-
ways of the same
polarity, that is, the
positive terminal,
say, is always posi-
p,o.8o tive; whereas, when

the converter is
brought up to speed
by allowing alter-
nating current to flow
through the arma-
ture, the terminal may
be positive at one
time, and the next
time the converter is
started it may show
a negative polarity.

When starting from
the alternating-cur-
rent side, the field is
unexcited and when
the current is first
thrown on, the volt-
meter connected to
the direct-current
fjo.80 Side will show no

Digitized by VjOOQIC

§26

AND MEASUREMENT

UJQQQQa>rKiHiiHm>^\iJQQQ^

wsjwviRraRr^^

Fio. 31

deflection because the E. M. F. between the direct-current
terminals is then rapidly alternating, and, hence, will not
effect a voltmeter of the Weston direct current or similar type
except perhaps to cause a trembling: of the needle. As the
converter comes up to speed, the frequency on the direct-cur-
rent side becomes slower and the voltmeter needle begins to
vibrate, its rate of vibration becoming: slower as the converter
g:ets more nearly into synchronism. At exact synchronism,
the E. M. F. on the direct-current side is steady; hence, the
voltmeter reading becomes steady. The field should be
excited just before S3mchronism is attained, and the polarity
of the direct-current ro^rension

terminals will depend b^b^

on which side of the
zero the voltmeter
pointer happens to
be when the field is
excited. If the ex-
citing switch is closed
with the pointer
on the wrong side,
the polarity will be Rurrnn^
wrong.

Another objection ^"""^
to starting with alter-
nating current is that
when the current first flows through the armature it sets
up an alternating flux through the field coils that may
induce extremely high E. M. F.'s in them. Since the field
coils are usually connected in series, the total E. M. F.
generated may be so high as to endanger the insulation
of the coils. When this method of starting is used, it is
customary to install a special switch for disconnecting the
field coils from each other while the converter is being started.
Just before synchronism is attained, the coils are connected
in the usual way and supplied with exciting current. It is
not usually advisable to apply the full alternating-current
voltage to the collector rings imtil the machine has come up

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

J^
^

to speed, because the full voltage will give rise to an objec-
tionable rush of current. To cut down the voltage at start-
ing, a starting compensator similar to that used in connection
with induction motors is suitable, but a simpler arrangement
is to bring out taps from the transformer secondaries and
connect these to a double-throw switch so that in one position
of the switch the converter receives half the secondary
voltage, while in the running position the full voltage is
applied. Fig. 31 shows this arrangement.

One considerable advantage in starting from the alternating-
current side is that the converter does not have to be synchro-
nized; it is^ brought
into synchronism by
the alternating cur-
rent. This is an im-
portant considera-
tion when a machine
must be started in
a hurry. Starting
from the alternating-
current side does not
give rise to undue
distiurbances if the
frequency of the
converter is fairly
low, say 25 cycles
per second. On many switchboards connections are pro-
vided so that the converters may be started with either
direct or alternating current.

When the converter is started from the direct-current side,
it is necessary to insert a resistance in the armature circuit.
Fig. 32 shows a type of starting rheostat used for this pur-
pose. On account of the unequal lengths of the switch clips,
the three sections of the resistance are successively short-
circuited as the switch is closed. As the converter starts
up as an unloaded direct-current motor, it comes up to
speed quite rapidly and a simple switch giving four or five
resistance steps is sufficient.

PlO.88

Digitized by VjOOQIC

§26

AND MEASUREMENT

Where direct current is not available, the converter may
be started by means of a small induction motor having its
armature mounted on an extension of the shaft. This method
is used by the Westinghouse Company. It involves the use
of a small auxiliary motor on each converter, and if the
station contained many machines it might be cheaper and
more satisfactory to install a small motor generator set and
start from the direct-current side.

29, Synclironiziiigr Rotary Converters. — Rotary con-
verters and synchronous motors are synchronized with the

Ma/n Bus ^wrs

AMr^mtch

-(>^

Sync^/ontzer

To Rotary
No2

Pio.38

line E. M. F. in the same way as an alternator is S3mchro-
nized with the bus-bar E. M. F. Lamps, voltmeters, or
synchronoscopes may be used to indicate the point of syn-
chronism. Fig. 33 shows a Lincoln synchronizer used to
indicate when either of two rotary converters is in syn-
chronism. In this case the converters are fed directly from
low-pressure bus-bars and potential transformers are not
needed in connection with the synchronizer. When the
pressure is more than 400 or 500 volts, potential trans-
formers should be used. Synchronizing lamps are also pro-
vided, enough lamps being" connected in series to stan4

Digitized by VjOOQIC

40 POWER TRANSFORMATION §26

the voltage. If converter No. 2 were to be synchro-
nized, plugs would be inserted at a, b, and ^, thus con-
necting the upper terminals of the synchronizer to the bus-
bars and the lower terminals to the corresponding phase
of the converter. When the synchronizer is used on pres-
sures somewhat above those for which it is made, it is
necessary to insert resistances as shown at r^ and r,. In
new installations, synchronoscopes are now used in pre-
ference to lamps.

APPARATUS FOR CONTROIiliING THE OUTGOING

CURRENT

30. The apparatus for the control of the outgoing
current is generally grouped on a switchboard by itself.
In most cases the current is delivered at comparatively
low pressure; hence, the devices used on the switchboard
for the outgoing current difiEer materially from those on
the incoming lines. Generally, the delivered current is
used for electric lighting and power, or street-railway pur-
poses, and the switchboard appliances used are the same
as if the power were supplied from an ordinary station.
Rotary converters are operated in parallel and connected
up on the direct-current side in exactly the same way as
direct-current machines. If they are compound wound an
equalizing connection must be used.

LOCATION AND GENERAL ARRANGEMENT OF
SUBSTATIONS

31, One of the greatest advantages of the distribution
of power by means of substations is that the substations
may be placed at or near the centers where the heaviest
demand for current exists. They do not have to be located
with reference to coal or water supply, and the price of real
estate becomes a comparatively small item, because substa-
tions have a very large output compared with the ground
space they occupy. They can' also be placed in location^

Digitized by VjOOQIC

§26 AND MEASUREMENT 41

where a i)Ower plant would not be permitted on account of
the smoke and dirt caused thereby. Substations can, for
these reasons, be placed near the center of the load, and
thus efiEect a great saving in the amount of copper required
for feeders.

32. Fig. 34 shows the interior, of a typical substation,
one of the substations in Buffalo, N. Y., supplied with
power from the Niagara power plant. All the machinery
and controlling devices are here placed in one room, and a

Fig. 34

single attendant is all that is needed. It is a fireproof
building provided with a hand-operated overhead traveling
crane for handling the machinery during installation, or in
case repairs are necessary. The step-down transformers
A, A are ranged along one side, and the three rotary con-
verters B, B, B along the other. Each converter is of 400
kilowatts capacity and is supplied by a group of three
150-kilowatt transformers, the secondaries of which are con-
nected to the converter; air-blast reactance coils, placed

4&— 30

Digitized by VjOOQIC

42 POWER TRANSFORMATION §26

behind the transformers, are inserted- between the trans-
formers and the converter in order to permit voltage regula-
tion by variation in field strength. The converters are six-
pole machines supplied with 25-cycle current, and run at a
speed of 500 revolutions per minute.

The incoming current at 10,000 volts enters in the base-
ment by means of a lead-covered cable and passes through
the hand-operated oil switch C, which is provided for cutting
oflE all power from the station in case of emergency or for
any other reason. From C, the current passes through the
high-tension circuit-breakers located on the switchboard Z>,
and provided with time-limit relays. After passing through
the circuit-breakers, the current goes to the high-tension
bus-bars E and from there to the three high-tension oil
switches F mounted in a brickwork casing. In the figure,
one of the iron covers is removed showing the three cells
of one switch. Each switch controls the current in the
primaries of a group of three transformers supplying a
rotary converter. The potential transformers for supplying
current to the voltmeters and synchronizing lamps are shown
at g, g on top of the oil switches. The switchboard for con-
trolling both the incoming and outgoing currents is shown
at H immediately below the gallery containing the high-
tension switches and circuit-breakers. The portion of the
switchboard that contains the instruments for the alternating
current is at the right-hand end at K\ three panels are
provided, one for each converter and group of three trans-
formers. The switch handles for operating switches F
are mounted on these panels and are thoroughly insulated,
by insulating joints, from the switches themselves. The
ammeters are supplied from current transformers, so that
none of the appliances on the switchboard with which the
operator might come in contact are exposed to the high
pressure; all the high-pressure devices are confined to the
upper gallery.

From the high-tension switches F, the current passes to
the primary coils of the transformers and the induced cur-
rent in the secondaries passes to the collector rings of the

Digitized by VjOOQIC

§26

AND MEASUREMENT

Digitized by VjOOQ IC

tf POWER TRAXSFORMTI05

bekDd Che trjasfuimgs, are msetcd bttwcec zi i

foracn ad the oooroter in order to pens± to^t -.^

Dob bf nnitxxi io Ud strcn^ The cosrnrts^ r-

pok 3urhrir< supplied wall ^croc carrea ai: r_

gxed uf W rtTohitxxis per nmmte.

The acommi cmrat at IO,(W) rolls eaters is re i
ax8t br mems of a Jead-comed abk aod passes ± i
Che handoperated oO switch C viiich is pronded ir l.i
off iIJ power from the statioo in case of emerfeacr - 1
«T other reasoQ. From C, tbt cnnent passes thrcv- ^
hxfh-tessxw drcnrt-breatos located ot die sc^tibarJ
jod pnwifcd with time-limit lelajs. After passing ±%i
the drcmt-hreaiers. the cnnent ^oes to die IcsiHSi:!
hos-hirs £ aod from there to the three kigiHe^ : i
switches f moosted io a bridnrori casing. Id ^ k^-'
oot oi the mn corers is remofed showing the dree ^•^■
oi oot switch. Each switch controls the cnrrent a 2
priotfries of a frotq> of three transformers snp{^' •
nxarj coorerter. The potentM transformers for sap^ I
carrest to the rohmeters and synchroniring lamps m ^sor.
Jt/i/'oo top 0/ the oil switches. The switchboard ftro-s^
trolte^ both die hKoming and outgoing cnrreots is shoe
Mt H fflunediate/f hdow the gallery containing the ^
aeBsioa switches aod cfrcm't-breaiers. The portwoflffc
swztchhoard tb^X cootaios the mstmments for die

Digitized by VjOOQIC

44 POWER TRANSFORMATION §26

converters. The direct current passes to the panels 1, 2, 3,
each of which is provided with a direct-current ammeter and
circuit-breaker in addition to the main switches. The out-
going feeders are connected to the feeder panels 4, 5, ^, etc.,
each of which is provided with an ammeter, circuit-breaker,
and main switch. Panel 9 carries an ammeter that measures
the combined output of the converters, a voltmeter for
measuring the direct-current voltage, and a recording watt-
meter for registering the output of the substation. The
voltmeter can be connected to any converter by means of
plug connections on each converter panel. The subbase of
each converter panel carries a single-pole switch for the
field, and a double-pole transfer switch for connecting which-
ever converter is to be started to the starting switch on
the subbase of panel 9, Each converter is provided with
an iron-clad magnet m mounted on the end of the bearing
casing. A current is sent through this magnet at regular
intervals, thus making the shaft oscillate back and forth
and keeping the brushes from wearing ridges in the com-
mutator. Mechanical devices that have the advantage of
not requiring any current for their operation have also been
designed for maintaining an oscillation of the shaft.

Fig. 35 shows the arrangement of a typical substation
for an electric railway. The arrangement of the trans-
formers, rotary converters, etc. is clearly shown, so that
further comment is unnecessary.

CONNECTIONS FOR SUBSTATIONS

33. The connections used for the various appliances in
a substation vary considerably in different installations, so
that it is impossible to give any scheme that is generally
applicable. For example, those for a substation supplying a
street-railway system will differ from those for one supplying
current for lighting purposes. In order to give an illustra-
tion of connections a few typical examples of substations for
supplying direct current will be selected. In the first case
the substation is to be supplied with current over one or

Digitized by VjOOQIC

§26 AND MEASUREMENT 45

both of a duplicate set of high-tension transmission lines.
Two compound-wound rotary converters are used, which
are to be arranged for parallel operation. The converters are
to be started by means of direct current supplied by either
one of the machines, it being assumed that one converter
is always in operation. In case both were shut down
for any cause, they could be started from the power station
by starting up the alternator and bringing the converters
and alternator up to speed together. Fig. 36 shows a
scheme of ^connections that might be used for such a sub-
station. It must be understood, however, that the connec-
tions in individual cases might differ considerably from
those shown, and yet give practically the same results.
The differences would not lie so much in the main connec-
tions as in those of the auxiliary parts, such as the various
instruments, synchronizing devices, etc.

34. Patb of Main Current. — The wiring, as a whole,
can be divided into two sections; that between the con-
verter 8 and the incoming lines 1, 2, and that between the
converter and the outgoing feeders 20, 20. In the first
section the current is alternating, while in the second it
is direct. The main current enters on either one or both of
the three-phase lines i, 2, and passes to the high-tension
bus-bars 5, 3. High-tension switches i' 2' are provided to
cut off all current from the station. From the bus-bars
3, 3, the high-tension current passes to the converters
through the switches 4, 4'. We will confine our attention
from this point to one converter, as the connections of
each are exactly alike. After reaching switch 4, the current
passes through the high-tension fuses 5 to the primary coils
of the step-down transformers 6. .The switch 4 is frequently
provided with an automatic tripping device that will open
the circuit in case of overload, in which case the fuses 5 are
not needed. In other cases a non-automatic switch is used
at 4, and automatic circuit-breakers instead of fuses at 5; the
transformers 6 step-down the line voltage to an amount
suitable for conversion. For example, in this case the

Digitized by VjOOQIC

46 POWER TRANSFORMATION §26

converters will supply a voltage of about 550 for street-rail-
way purposes, and the voltage supplied by the secondaries
of 6 will, for a three-phase converter, be 550 X .612 = 337
volts, approximately. From 6 the low-pressure alternating
current passes through the reactance coils 7, which are
inserted to allow voltage regulation; in case potential regu-
lators are used instead of reactance coils, they are inserted
at this point. From 7, the current passes to the collector
rings of the converter 8 and is transformed to direct current
at 550 volts. The direct current passes through the main
switches 11, IP to the direct-current bus-bars 14. Since this
substation supplies an ordinary street railway operating with
an overhead trolley or third rail, the negative bus-bar is con-
nected to the track and ground, while the positive connects
to the outgoing feeders, which in turn are attached to the
trolley wire*or third rail, as the case may be.

35. Connections for Synclironizing:. — Each of the
incoming lines is provided with a potential transformer
/' or /", and each converter is also provided with a high-
tension transformer, such as /'^' connected between the
switch and the transformer primaries. In series with the
secondaries of each transformer is a synchronizing lamp
/,, /„ etc. Suppose that current is being supplied over line 1
and that converter 8 is to be synchronized. The converter
is started, switch 4 being open, by supplying it with direct
current. It generates an alternating current that is stepped-
up by transformers 6 and supplies the primary of /'" with
an alternating E. M. F. By inserting plugs at a and c the
secondaries at /' and /'" are connected in series with each
other and with lamps / and /,. If one plug c is cross-con-
nected, as indicated by the dotted lines, the lamps will be
bright at synchronism. The synchronizing arrangement is
essentially the same as that described in connection with
the operation of alternators in parallel.

36. Voltmeter Connections. — In order to obtain a
reading of the voltage on either incoming line, a voltmeter V
is provided. By means of a voltmeter plug, connecting the

Digitized by VjOOQIC

§26 AND MEASUREMENT 47

upper and lower terminals of either of the receptacles e, /,
the voltmeter can be made to indicate the voltage on
either line. The voltage of the high-tension side of either
converter can be measured by means of the voltmeter F',
which is connected to the voltmeter receptacles g, h. The
voltage of the direct-current side of the converters is indi-
cated by the voltmeters O, O connected to the voltmeter
receptacles p^p^. The voltage of a converter can thus be
compared with the voltage of the line or direct-current
bus-bars to which it is to be connected.

37. Ammeter Connections. — Each converter is pro-
vided with an ammeter / connected to the secondary of a
current transformer inserted between the switch 4 and the
transformer primaries. In some cases an ammeter is inserted
in each line wire, especially in large installations, though
this is not absolutely necessary. In some cases, also,
ammeters are placed on the incoming lines, series-trans-
formers, of course, being used so as to thoroughly insulate the
instruments from the high-tension line. The direct-current
side of each converter is provided with an ammeter 21 con-
nected across a shunt 12. Ammeter C indicates the total
direct current, since its shunt is connected in series with the
main bus-bar between the converters and the feeders. The
feeders are provided with feeder ammeters t, V connected
across the shunts 19, 19'.

38. Circuit-Breakers. — In this case the incoming lines
are not equipped with automatic circuit-breakers, though, if
the substation formed part of a large network, circuit-
breakers would likely be inserted at kk', and these would
be equipped with reverse-current and time-limit attachments.
On the direct-current side each converter is provided with a
circuit-breaker 13, 13' connected between the converter and
the direct-current bus-bars. Each feeder is also provided
with a circuit-breaker, as indicated at 18, 18',

39. Equalizer Connection. — The positive brushes of
the converters are connected by means of an equalizer cable
in which the equalizer switch 15 is inserted. Note that

Digitized by VjOOQIC

48 POWER TRANSFORMATION §26

the equalizer connects the two brushes to which the series-
field windings are attached.

^0. Shunt-Field Connections. — One end of the shunt
field connects to the -|- brush, and the other to one terminal
of the field rheostat R. The other rheostat terminal con-
nects to the blade of the field switch m. When switch m is
moved to the right, thus cutting the current off from the
shunt field, the pilot lamp /«, resistance r, and rheostat R are
connected across the field terminals, thus allowing the
induced E. M. F.', caused by the interruption of the field
current, to discharge through this closed circuit. Switch m
is in the position shown in the figure when the converter is
in operation. Switch n allows the shunt field to be excited
either from the direct-current bus-bars or from the con-
verter itself. When it is partly closed, the blade makes
contact with the long clip and the field is excited from the
bus-bars; when fully closed, the field is connected across
the brushes.

41. Method of Starting:. — Suppose converter 8' is in
operation supplying current to the direct-current bus-bars,
and that 8 is to be started and thrown in parallel with <5*'.
Switches 4, 11, 11\ 15, n, and m are supposed to be open.
Close the equalizer switch 15] place field switch m in the
position shown in the figure, and close switch n until the
blade makes contact with the long clip. The shunt field will
then be excited by current from 8\ because one end of the
field is connected through R, m, and n to the negative bus-
bar, and the other end is connected to the positive side of &
through the equalizer. Close switch ii, thus allowing ciw-
rent to flow through the series-coils .9. The field is now
fully excited and the converter can be started as a direct-
current motor by allowing current to flow through its arma-
ture. This is done by throwing the switch s to the upper
position and gradually closmg the starting switch 5*. The
speed of 8 can be adjusted by moving the field rheostat R,
and when the point of synchronism is attained, as indicated
by the synchronizing lamps, switch 4 is closed. After S has

Digitized by VjOOQIC

I I II 1

C Btta-hoi^

eqy0iizlnq Bua

Digitized by VjOOQIC

\}i^tmd^^^&yMsssij

^RRryijOTwyOTOTir

Rm^efmn^^

'^I^

Starftft9 Swifch ^ _

t T »

220

Digitized by VjOOQIC

§26 AND MEASUREMENT 49

been closed and the resistance cut out, switch 11' should be
closed and switches 5, s opened; also, n should be fully
closed, thus connecting the shunt field across the terminals
of the converter and allowing the field to remain excited
even if switches 11, 11\ and 15 are open. The transfer
switch s is provided so that the starting rheostat S can be
connected to either converter.

This method of starting from the direct-current side is
sometimes modified as follows: The converter is speeded
up as before and the field rheostat is adjusted so that the
machine runs somewhat above synchronism. Then switches
lly 11', and n are opened, thus cutting off the direct current
and opening the field circuit. The converter is then running
above synchronism under its own momentum, but is genera-
ting no E. M. F. Switch 4 is then closed and the converter
is brought into synchronism by the alternating current, and
as it is already running at nearly synchronous speed the
amount of current required is not nearly as great as if the
converter were started from rest by allowing alternating
current to flow through the armature. The field circuit is
then closed, the direct-current voltage adjusted, and the con-
verter thrown in parallel on the direct-current side in the
usual manner. This method of starting is sometimes
advantageous when the load on the direct-current bus-bars
is of a very fluctuating nature. The variations in voltage
may under such circumstances make it difficult to syn-
chronize with the lamps in the ordinary way.

In case the converters are started by means of an auxiliary
induction motor mounted on the shaft, switches 5*, s are
omitted and the necessary connections for the starting motor
are provided instead.

42. Fig. 37 shows connections for a substation contain-
ing two rotary converters supplying current to a two-wire
lighting or power system. The connections are, on the
whole, very similar to those just described but differ from
them in minor details. The switchboard is divided into two
parts — the alternating-current board at the right and the

Digitized by VjOOQIC

60 POWER TRANSFORMATION §26

direct-current board at the left. The alternating-current
board consists of two panels, each of which is equipped
with a main switch, which may be located some distance
from the panel but yet be operated therefrom; a voltmeter,
ammeter, power-factor indicator, overload relays, synchroni-
zing lamps, synchronizing plug, and potential, and current
transformers. Each direct-current panel is equipped with
two single-pole main switches, field rheostat, machine
ammeter, circuit-breaker, voltmeter plug, and starting
switch for starting from the direct-current side. Each feeder
panel, of which one is shown in the figure, is equipped with
a double-pole feeder switch, feeder ammeter, circuit-breaker,
and lightning arrester. In addition to the instruments on
the generator and feeder panels, a total output ammeter and
a total output recording wattmeter are connected between
the converters and feeders so as to measure the combined
output of the machines. Also, two voltmeters are provided —
one to indicate the bus-bar voltage and the other to indi-
cate the voltage of the direct-current side of either converter.
These instruments, together with the total output meters,
are often moimted on a panel by themselves.

It will be noted in Fig. 37 that the connections are such
that the converters can be started from either side. Each
machine is proviaed with a double-throw starting switch on
the alternating-current side by means of which the converter
is supplied with a reduced voltage at starting. The primaries
of the transformers are provided with a number of taps to
adapt them to different line voltages, and reactance coils are
inserted between the secondaries and the collector rings.
The main switch is provided with an automatic tripping
attachment that is operated by the overload relays. The
synchronizing connections are such that either the synchro-
nizing lamps or voltmeter may be used. Each converter
is equipped with a power-factor indicator, which shows
whether the current taken from the bus-bars is lagging or
leading. The operation of this type of power-factor indi-
cator will be explained later after polyphase meters have
been taken up.

Digitized by VjOOQIC

T=t

Machine
CurcuitBremk€r

fftt * • Y (Ammeter ^

Waffmefer

Switch.
'U^htmnfArres

thtf

^ekfBreaA'idp Switch.

?26

Digitized by VjOOQIC

§26 AND MEASUREMENT 51

43. The method of starting from the direct-current side
is briefly as follows: On the alternating-current side, the
starting switch is thrown to the lower position and the main
oil switch is open. The field break-up switch and the equal-
izer switch at the machine are also" closed. The break-up
switch is used only when the converter is started from the
alternating-current side. The + main switch, the circuit-
breaker, and the single-pole starting switch are then closed,
first making sure that the starting rheostat switch is open.
Closing the + main switch and the equalizer switch places the
series-coils in parallel with the series-coils of the converter
that is already in operation and also connects one end of the
shunt-field winding to the -f side of the system. As soon as
the starting rheostat switch is placed on the first point, cur-
rent flows through the armature and shunt field. The con-
verter then starts as a direct-current motor and comes up to
speed as the starting rheostat switch is pushed in. After
this switch has been fully closed, the — main switch is
closed and the rheostat switch opened. The converter is
now synchronized by varying the field strength, and when
the lamps or voltmeter indicate synchronism the oil switch
is closed.

When a converter is started from the alternating-current
side, the switches on the direct-current side are open and the
field break-up switch is also open. The double-throw start-
ing switch is thrown to the upper position and the main oil
switch closed. When the machine has attained speed, the
starting switch is thrown over to the full-voltage position.
The field is then excited and, after making sure that the
polarity of the direct-current side is correct as indicated by
the direct-current voltmeter, the converter is thrown in par-
allel on the direct-current bus-bars.

44. Fig. 38 is a diagram of connections similar to Fig. 37
except that, since the direct current is delivered to a railway
system, the arrangement of the apparatus on the direct-
current side is different. The connections on the alternating-
current side are shown for one converter only; they are the

Digitized by VjOOQIC

52 POWER TRANSFORMATION §26

same as in Fig. 37. The negative bus-bar is placed near the
machines instead of on the direct-current switchboard, and
the negative main switch is placed alongside the equalizer
switch, the converters being equalized on the negative side.
The negative bus-bar is connected directly to the rail or
return circuit, so that the direct-current panels are single-pole
and the connections thereby simplified. The arrangement
shown in Fig. 38 is used by the General Electric Company
and the direct-current ammeters are of the Thomson astatic
type, in which the magnetic field is supplied by electromag-
nets excited from the bus-bars. Each ammeter has a pair of
wires to supply the exciting current in addition to the usual
pair connecting to the ammeter shunt. The series-field of
each converter is provided with a shunt to regulate the
amount of compounding; this shunt can be cut out by means
of the switch shown in the figure. This is necessary when
starting from the alternating-current side; otherwise, the
alternating E. M. F. induced in the series-coils would set up a
large current through the shunt. These diagrams give a
general idea of the connections used for substations, but it
must be remembered that they admit of considerable varia-
tion and must be adapted to the requirements of each particu-
lar case. It is not possible therefore to lay down any general
scheme that is applicable to all cases.

Digitized by VjOOQIC

§26 AND MEASUREMENT 53

MEASUREMENT OF POWER ON POLY-
PHASE CIRCUITS

INSTRUMENTS USED FOR POWER MEASUREMENT

45. Reference has already been made, in connection with
alternating currents, to the measurement of power on alter-
nating-current circuits. The measurements there described
related to simple single-phase circuits; the influence of the
power factor on the actual power delivered was pointed out,
and the use of the wattmeter was explained. As the appli-
cations of polyphase currents to power transmission have
now been described, it will be advisable to consider the
methods available for measuring the power supplied to two-
phase and three-phase systems.

46. On account of the fact that the power factor of
alternating-current circuits, either single-phase or polyphase,
is seldom 100 per cent. 6t unity, power measurements are sel-
dom made with ammeters and voltmeters as in direct-current
work. The three ammeter and three voltmeter methods are
inconvenient, liable to considerable error, and are never used
if wattmeters are available. Good portable wattmeters
are now obtainable at a price but little greater than that of
ammeters or voltmeters. The wattmetef does not indicate
the product of the volts and amperes, but the product,
volts X amperes X cos <f>y where cos <^ is the power factor.

In making practical power measurements we may wish to
obtain simply a reading of the total watts supplied at any
given time or we may wish to obtain the total work done,
in watt-hours or kilowatt-hours, during a certain period of
time. In the first case, indicating wattmeters would be used
to make the measurements, while in the second it would be
necessary to use recording wattmeters, or watt-hour meters,
as they should more properly be called.

Digitized by VjOOQIC

54 POWER TRANSFORMATION

INDICATING WATTMETERS

47. The indicating wattmeters used for power meas-
urement on polyphase circuits are in nowise different from
those ah-eady described for use on single-phase circuits.
Many reliable makes of portable wattmeters are now avail-
able and these are used for commercial measiurements. The
number of wattmeters required for a given test depends on
the conditions under which the test is made. In some cases
one wattmeter is sufficient; in others, two are necessary,
as will be shown. In connection with polyphase measure-
ments, it is well to bear in mind the fact that if the differ-
ence in phase between the currents in the two coils of a
Siemens type of wattmeter becomes more than 90°, the
twisting action on the movable coil reverses, and, hence
the deflection reverses. In ordinary single-phase circuits
this condition does not arise, but it is possible in certain
cases to have a greater phase difference than 90° on
three-phase circuits, and the negative deflection referred
to above must be taken into account.

RECORDING WATTMETERS

48. The Thomson recording wattmeter has been
described; it operates on either direct or alternating current
and can be used for measurements on polyphase or single-
phase circuits. Meters of the induction type, having no
commutator, are simpler in construction than the commu-
tator meter, and have rapidly come into favor. They, of
course, have the disadvantage that they cannot be used
on direct current, whereas the Thomson meter cah be used
on either direct or alternating, a considerable advantage
where a company supplies both kinds of current. Also,
induction meters must be used on circuits having the fre-
quency for which they are adjusted; if used on circuits of
other frequency their indications will be incorrect.

49. Induction wattmeters are made in many different
forms, but they all operate on about the same principle.

Digitized by VjOOQIC

§26

AND MEASUREMENT

They are essentially small induction motors designed to
operate with single-phase or polyphase current. Figs. 39
and 40 illustrate the operation of this class of recording
meter, though it will be understood that it is possible to have
a different arrangement of the parts and yet have the meter
operate equally well.

fbtenhaf
Cot/ —

Core

Current
dot/-

core.

In Fig. 39, a is a coil of
fine wire wound on the
laminated iron core b\
Cy c are coils of a few
turns wound on a core d^
which is entirely sepa-
rate from/. An alumi-
num disk e is mounted
on the shaft / so that the
outer part of the disk
revolves past the ends
of the cores on which
the coils are wound.
Fig. 40 shows a section
of the coils and core
taken along the line ig.
Coils c, c are connected
in series with each other
and with the circuit so
that all the current sup-
plied passes through
them. The potential
coil a is connected across
the circuit so that the
current in it is propor-
tional to the voltage;
c and a therefore corre-
spond to the current and potential coils of an ordinary watt-
meter. The magnetism set up in core b will be proportional
to the voltage, and that set up in core ^ will be proportional to
the current. Coil a has a high inductance and an additional
inductance is usually connected in series with it; in any event,

Fig. 39

Digitized by VjOOQIC

56 POWER TRANSFORMATION §26

the meter is so desig^ned that the current in coil a will
lag approximately 90° behind the E. M. P., thus making the
magnetism in * lag 90° behind the E. M. F. The current
in coils r, c is, of course, in phase with the current supplied
to the circuit in which the meter is connected. The alter-
nating magnetic field set up, say, by coil a induces eddy cur-
rents in the disk, which spread out somewhat as indicated by
the dotted lines o. Fig. 39 (^). These currents are reacted
on by the field that emanates from the poles of core d and
the disk is made to rotate.

In order that the meter shall give an accurate indication of
the work done in the circuit, the driving torque on the disk
must be proportional to EI cos <l>, where cos <l> is the power
factor of the circuit. Let us first consider the case where the

power factor is 1, i. e.,
where the line current and
line E. M. F. are in phase.
The current in a is at right
angles to the line E. M. F.
and the induced eddy cur-
rents in the disk are at
right angles to the mag-
netic flux, because these
currents depend on the rate of change of the flux, and the
flux is changing most rapidly when the magnetizing current is
passing through zero. The magnetism in flf is in phase with
the current; hence, for the power factor of 1, the currents in
the disk are in phase with the magnetism set up by the series-
coils; consequently, the driving torque is a maximum for the
given values of the line current and E. M. F. Suppose that
we have the same current and E. M. F. but that the power
factor is less than 1. The line current will lag behind
the E. M. F., the magnetism in a? will not reach its maximum
at the same instant as the currents in the disk, and the
driving torque will be reduced, thus making the meter run
slower. A magnetic brake is provided by making the disk
revolve between the poles of permanent magnets in the same
manner as in the Thomson meter. This makes the speed at

Fig. 40

Digitized by VjOOQIC

§26 AND MEASUREMENT 57

all times proportional to the driving torque. If it were
possible for the circuit to have a power factor of zero, i. e., if
the line current lagged 90^ behind the E. M. F., the torque
action on the disk would be zero, because the induced cur-
rents would be at right angles to the magnetism in d. In
other words, when the currents in the disk were a maximum
there would be no field for them to react on, and when the field
magnetism was at its maximum there would be no currents in
the disk. The meter would not therefore record any power
even though current would be flowing in coils a, c. This is

Fig. 41

as it should be, because with zero power factor, the watts
supplied would be zero no matter what the values of the cur-
rent and E. M. F. might be. The induction meter can there-
fore be made to record the number of true watts expended in
a circuit no matter what value the power factor may have.

50. Fort Wayne Induction Wattmeter. — Fig. 41
shows a Fort Wayne single-phase induction wattmeter.
D is the armature, which, in this meter, takes the form of
an inverted aluminum cup. E is the damping magnet that
exerts a drag on the armature and makes its speed pro-
portional to the driving torque. The current and potential

45—31

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

coils are at the back of the armature; a is one current coil,
and the other coil occupies a similar position on the
opposite side of the armature. The speed of the meter can
be adjusted by shifting: the magnet E up or down, thus vary-
ing the amount of the armature embraced by the pole pieces
of the permanent magnet.

51. Stanley Induotlon Wattmeter. — In the Stanley
recording wattmeter the armature is an aluminum disk acted
on by current and potential coils in much the same manner as
previously described. The most interesting feature of this

meter is the method
of suspending the
disk. Instead of rest*
ing on a pivot, as
in most meters, the
disk a, Fig. 42, is sus-
pended magnetically.
It is mounted on a
small, hollow, steel
shaft b through which
passes a fine steel
wire c stretched taut
by means of the screw
d and spring e. The
sKaft b has in it two
small brass bushings, one at each end, that bear against the
wire and keep the disk from tipping sidewise, otherwise the
disk has no support. A permanent magnet / is provided with
pole pieces gy h shaped as shown; ^ is a brass plug. From
the way in which the pole pieces are shaped the lines of force
passing across the gap at / hold the shaft in a central position
between the poles so that the shaft and disk are magnetically
suspended and revolve with very little friction. The reduc-
tion in the friction makes the meter more accurate, particu-
larly on light loads, and there is no pivot to be damaged by
shock or vibration. The recording dial is operated by gears
driven from the shaft by the teeth shown at m.

Digitized by VjOOQIC

§26 AND MEASUREMENT 69

MEASUREMENT OP POWER ON TWO-PHASE
CIRCUITS

52. In making power measurements on polyphase cir-
cuits, the methods used will depend, to some extent, on
whether the load on the system is balanced or not. The
load in such a system is said to be balanced when the current
in each of the phases is alike, and the power factor of the
load on each phase also alike. In other words, the loads on
the different phases of a balanced system are alike in every
particular; under such circumstances it would be accurate
enough to simply measure the power delivered to one phase
and multiply the result by the number of phases. Unfortu-
nately, an exact balance is seldom realized in practice,

CurnptCoil

f>^^ V^ii^SPtUi

l&lme, Td l^ad

P/kxseS.

^•-*^ QOPOQ i— ^

xs^mj

Pio. 48

although induction motors, synchronous motors, and rotary
converters in themselves constitute a nearly balanced load,
because they take current from the different phases in
practically equal amounts. When a mixed load of lights
and motors is operated, it is almost impossible to obtain an
exact balance.

63. Two-Phase, Four- Wire System. — Fig. 43 shows
the usual method of connecting wattmeters for raeasming
power on a two-phase, four-wire system. Each phase is
provided with a wattmeter, there being a current coil in each
phase; the pressure coils are connected across the phases.
In series with the pressure coil there would be a resistance,
as in all wattmeters of the electrodynamomater type; this

Digitized by VjOOQIC

60 POWER TRANSFORMATION §26

resistance is not shown in the accompanying figures, and
the fine-wire coil can be taken to represent the complete
potential circuit of the wattmeter including the usual pro-
tective resistance.

Fig. 43 shows two distinct circuits containing wattmeters.
It is evident that the sum of the two readings will give the
total power supplied to the motor or other devices to which
the lines are connected. Also, the sum of the readings will
give the power supplied whether the load is unbalanced or
not, because each wattmeter measures the actual number
of watts supplied to the phase in which it is connected.
Fig. 44 shows the two wattmeters used on a two-phase
system with a common return. Recording wattmeters of

nmm

/^>cxse/

P/iaseP

O.Q.QQQ.OQ/

Jiftflflftj

Pig. 44

the induction type are made, in which two sets of series-
coils and two potential coils act on a common armature,
thus practically combining two single-phase meters into a
single meter, so that only one instrument is required to
measure the energy no matter what the power factor may be
or how unbalanced the current in the two phases.

54. Induction Wattmeter for Unbalanced Poly-
phase Circuits. — Fig. 45 shows a General Electric poly-
phase meter of the induction type for measuring energy
supplied to unbalanced two-phase, three-phase, or monocyclic
circuits. It operates on exactly the same principle as the
single-phase induction wattmeter and is essentially two
sets of single-phase meter coils acting on a common disk
armature a. The two potential coils b, b are shown above
the disk; they are connected in series with the reactance

Digitized by VjOOQIC

§26 AND MEASUREMENT 61

coils ^, c. There are four current coils, two of which are
shown at d^ d. A pair of current coils is situated under
each potential coil and current is supplied to the front pair
by means of the conducting strips e, e. The ends of the
series-coils connect to terminals /, /, g, g, to which the
mains are connected; h is one of the t)vo magnets that
retard the disk. Each set of coils b, d, d constitutes a
single-phase induction meter, and as both these act on the
same disk a, it follows that the resultant effort that ttuns

Pig. 45

the disk is a combination of the efforts exerted by the two
sets of coils, and the record given by the meter is, therefore,
a true indication of the watts supplied. In Fig. 45, one set of
series-coils dy d would be connected in series with phase 1, and
the other set in series with phase 2. The potential coils b, b
would be connected across the two phases. In a three-phase
circuit the two sets of series-coils would be connected in series
with the two outside wires, and the potential coils would be
connected between the outside wires and the middle wire.

Digitized by VjOOQIC

62 POWER TRANSFORMATION ' §26

BSm Use of a Single Wattmeter on Trvo-Pliase
Circuit. — Figs. 46 and 47 show two methods of measuring
the power on a two-phase circuit with a single wattmeter;
these can be used in case the load is balanced. In
Fig. 46, the current coil is connected in the common
return wire, and a reading is first taken with the poten-
tial coil connected across phase 2, as shown by the
full line. The connection a is then transferred to a\ thus
connecting the potential coil across the other phase. The

H^OOCffN 7bZA«/

Pio.46

sum of the two readings gives the total power supplied
no matter what the power factor of the load may be. In
Fig. 47, the potential coil is connected across the outside
wires, while the current coil is connected in the middle wire.
The reading of the wattmeter gives the total number of
watts because, if the system is balanced, the resultant cur-
rent will differ in phase from the resultant E. M. F. by the
same amount that the current in each phase differs from

P/Kfse/

^^WS^

f)^Hfse2 rsSiSiSmSLr^

-♦72? load

Pig. 47

the E. M. F. of each phase. The resultant E. M. F., i. e.,
the E. M. F. E' between the outside wires, is '^ E, where E
is the E. M. F. of each phase. The resultant current /', i. e.,
the current in the middle wire, is ^ /, where / is the current
in each phase. The reading of the wattmeter is E' /' cos <^,
where ^ is the angle of lag. The watts supplied to phase 1
are \E I cos ^ and the same to phase 2, so that the total
watts supplied are 2 ^ / cos <^. Now E^ ^ ^ E and P
■ -^ /; hence, E' P cos <^ = V2 £ V2/cos* = 2-£'/cos*.

Digitized by VjOOQIC

§26

AND MEASUREMENT

That is, a single wattmeter connected as shown in Fig. 47
indicates the total number of watts supplied provided the
load is balanced. These methods of using a single watt-
meter are convenient, but it is always best to use the two
wattmeters if they can be obtained, because one cannot
always be certain that the load is balanced.

MEASUREMENT OP POWER ON THREE-PHASE
CIRCUITS

66. Power may be measmred on a three-phase circuit
by using one, two, or three wattmeters. Two-wattmeter
measurements are the most common, as the use of a single
wattmeter requires either that the load be exactly balanced,
or that the connections be transferred from one phase to
another and the load kept constant during the change.

Fig. 48

67, Use of Tliree Wattmeters. — Let y^ ^ C Fig. 48,
represent the three windings of a Y-connected three-phase
alternator. In a balanced system, ^„ ^„ and e^ being equal,
the line E. M. F.*s E^, E^, E^ are also equal, and are equal
to the E. M. F. in one winding multiplied by Vs. The cur-
rent in each line will be the same as the current in the winding
to which it is connected, and in a balanced system the three
currents will be equal. Three wattmeters with their current
coils A' B' C connected in the lines and their potential
coils a b c connected across the corresponding winding,
will measure the power delivered no matter whether the
load be balanced or unbalanced, inductive or non-inductive.

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

It is evident from the way in which the wattmeters are
connected that the potential applied to the pressure coil is
equal to that generated in the winding with which the cur-
rent coil is in series. Hence, the reading of wattmeter A^
will be €x ix cos <^, where </> is the angle of lag between the
current and E. M. F. The other two meters will give the
power developed in phases By C, and the sum of the three
readings gives the total power. If the load were exactly
balanced it would be necessary to use but one watt-
meter and multiply its reading by 3 to obtain the total
power. In case ABC represented the windings of an induc-
tion motor, synchronous motor, transformers, or in fact a
load of any kind, this method of measuring the power could

^WOO^

Fig. 49

be applied, though, as shown later, it is possible to measure
an unbalanced three-phase load with two meters, and the
three-meter method is therefore little used.

In most cases it is not possible to get at the neutral pointer.
Fig. 48, to connect the potential coils. In such Qases an
artificial neutral point may be obtained, as shown in Fig. 49,
by connecting three non-inductive resistances x, y^ z across
the three phases, and attaching their neutral point x' to the
potential coils. These resistances might be made up of wire
wound non-inductively, or of incandescent lamps. The sum
of the three wattmeter readings would then give the total
power supplied as before.

Digitized by VjOOQIC

§26

AND MEASUREMENT

58. Use of Single Wattmeter Wltli Y Resistance.

If the load were balanced, it would be sufficient in Fig. 49
to use but one wattmeter and multiply its reading by 3.

CarrmfCoit.

[ /^(^tfoo^

^///7^

f^tassurm Calf

y /^ejdsAoTKe.

Pig. 50

Figf. 50 shows the connections for a single wattmeter used
in this way. The resistances r,, r, correspond to resistances
x^ 2. Resistance r, is the usual protective resistance in
series with the movable ^

wattmeter coil. Fig. 51
shows a Thomson re-
cording wattmeter with
Y resistance; a is the
starting coil of the watt-
meter intended to com-
pensate for the friction
and to secure more ac-
curate readings on light
loads. By comparing ^TS
Figs. 50 and 51 it will
be seen that the connec-
tions are identical, the
recording meter being
connected in exactly
the same way as the-
indicating instrument.
Fig. 52 shows the con-
nections of a recording
meter on a three-phase balanced circuit where the pressure
is over 500 volts; the potential circuit is here supplied
through the small step-down transformers /, /. For very

Fig. 51

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

high-pressure circuits, the current coils would be connected

to the secondaries of
current transformers in-
stead of directly in the
circuit. Fig. 53 shows
the connections of a
Wagner indicating watt-
meter for measuring
the power on a balanced
three-phase circuit.
The stationary current
coils A, A are connected
in series with the sec-
ondary of a current
transformer C instead
of being placed directly
in the circuit. The
movable potential coil
B is supplied with cur-
rent from the small
transformers Z>, D. A
Y resistance is used,
the two branches being
in the separate cage £;
to limit the current in

Pio. 62

the protective resistance F is used
the potential coil.

59. Use of Two Wattmeters on Three-Pliase Cir-
cuits.— The most common method of measuring the power
supplied to a three-phase circuit is by means of two watt-
meters connected as shown in Fig. 54. The current coils Ay B
are connected in two of the lines, and the potential coils
between these two lines and the third line. If the power
factor of the load is over .5, i. e., if the angle of lag is less
than 60°, the sum of the two wattmeter readings gives the
power supplied. If the power factor is less than .5, i. e., if
the angle of lag is greater than 60°, the diflEerence of the
readings gives the power.

Digitized by VjOOQIC

§26

AND MEASUREMENT

Since the coil A, Fig. 54, is connected in one line and the
potential coil a between the outside and middle lines, it is
evident that even on a non-inductive load the current in -^4 is
not in phase with the current in a. On a non-inductive load

A/VNA/O
JB

VAAAAO

-#VVVA^ '

Pio.58

the current in A will differ in phase from the E. M. F. between
1 and 2 by 30°, and the current in B will differ in phase from
the E. M. F. between 2 and 5 'by 30°. In Fig. 55, suppose
that X represents the neutral point of the system, and that the
lines x~l\ x-2\ x-3' represent the three voltages, differing

Tbj^UL.

f^
^

j^issO^

Pio.54

in phase by 120°. Then the voltage between lines 1 and 2
is equal to V-x plus 2'-x, and is found by producing x-2*
backwards and finding the resultant ^-4. This resultant
is 30° behind the voltage x-V. The voltage between lines

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

2 and 3 is x-S' found by producing x-^' backwards and
combining with x-2'. Since the wattmeters are connected
symmetrically, as in Fig. 54, we must consider the E. M. F.
acting on coil b as the E. M. F. taken in the direction 3-2
or 3^-2' instead of 2'-^5^ since we have taken the other
E. M. F. in the direction i-2 or 1^-2'. The E. M. F. act-
ing on coil b will therefore be represented by v-6^ equal
and opposite to x-5'. Now, when the load is non-inductive,
the current in coils A and B is in phase with the E. M. F.*s
x-1' and x-3', so that the E. M. F. acting on coil a is "30®
behind the current in a, and the E. M. F. acting on b is
30° ahead of the current in B.

Pro. 66

If the load is inductive, the currents in the coils Ay B
instead of coinciding in phase with x-1' and x~3' will lag by
an angle 4>, cos 4> being equal to the power factor of the
load. The current will then be represented by the lines x-Ix
and x-I^ lagging <^ degrees behind x-1' and x-3'f Lines X'-4
and x-6' represent the E. M. F.*s applied to coils a, b so that
with an inductive load the phase difference between the
currents in A and a is 80° — <^, and between the currents
in B and b it is 80° + <^. If we represent the pressures x-l\
x-2\ x-3'y etc. by e, the pressure x-4 or the line pressure
will be V3 e. The watts indicated by A will be Vs e /» cos

Digitized by VjOOQIC

§26 AND MEASUREMENT 69

(30° -</>), and the watts indicated hyB,^[Se /. cos (30° + *) ,
and the sum of these two readings gives the power.*

60. It is now easily seen why a power factor of less than
.5 will give a negative reading on one of the wattmeters.
If the lag is 60°, the current in B differs in phase from that
in ^ by 30 + </> = 90°; no effort is exerted on the swing-
ing coil of the wattmeter and no deflection is given. If
the lag becomes greater than 60° a torque is exerted
in the reverse direction on the movable coil, and a
negative deflection is obtained. For power factors greater
than .5, both wattmeters will give positive readings, but
their readings will not be alike and both positive unless <l>
becomes zero, i. e., unless the power factor is 100 per cent,
or unity. If the angle of lag becomes 90°, both wattmeters
will read alike, but one will be positive and the other nega-
tive, so that their sum will be zero. This is as it should be,
because when the lag is 90° the current flowing in the circuit
is wattless and no power is expended. The conditions under
which the test is made will nearly always indicate whether
or not a negative reading is to be expected. If there is any
doubt on the matter, connect the meters to a load of lamps
and after all connections have been made so that both meters
read properly, take off the lamps and connect the load under
test. If one of the meters gives a reverse reading it shows
that the reading is negative and that the difference in the
two readings must be taken to give the number of watts sup-
plied. Fig. 56 shows the connections of a Wagner indicating

*That the sum of these two readings gives the power is easily shown
for the case of a balanced circuit where A = /,. We have, power
= IV = ij3 e A cos (30*' - 0) + >/3 e /, cos (30° + ^) . From trigo-
nometry we know that cos (30** -h ^) = cos 30° cos ^ — sin 30° sin ^,
and cos (30° - ^) = cos 30° cos <p -h sin 30° sin ^. Substituting these
values for cos (30° -h ^) and cos (30° — ^) , we have

JF = 2 V3 € A cos 30° cos ^,

V3
but cos 30° = -^; hence, W^ = 3 e /, cos ^, but £ /, cos ^ is the power

in one phase, and 3 c/, cos ^ is the total power, so that the sum
of the two wattmeter readings gives the total power supplied to the
circuit.

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

wattmeter for measuring the watts on a three-phase circuit
with balanced or tmbalanced loads and with any power
factor. It consists essentially of two wattmeters; A A, A' A'
are the two sets of current coils and B, B' the two movable
potential coils mounted on the same shaft. The torque
due to the two wattmeters is thus added or subtracted, as the
case may be, and the pointer attached to the shaft indicates
the actual number of watts expended. The current coils are
supplied from current transformers, and each of the movable
coils has a non-inductive resistance in series with it. This

Pio.56

wattmeter is also suitable for measurements on an unbalanced
two-phase system.

The recording wattmeter, shown in Fig. 45, is used largely
for measurements on three-phase circuits. Since the two
wattmeter elements act on a common armature, if one of
them gives a negative turning effort, the net turning effect
on the disk is reduced and the record on the dial is due to
the difference of the effects of the two wattmeters. The
instrument, therefore, gives an accurate record, no matter
what the power factor may be.

Digitized by VjOOQIC

§ 26 AND' MEASUREMENT 71

61. Measurement of Po^wer Factor. — The fact that
the ratio of the two wattmeter readings, Fig. 54, varies with
the power factor of the load affords a method of determin-
ing the power factor from the wattmeter readings.* Of
com-se if ammeter and voltmeter readings are available
the power factor can be calculated, since it is equal to

: — - the true number of watts being obtained from

apparent watts

the wattmeter readings and the apparent watts from the
voltmeter and ammeter readings. For a three-phase circuit
the apparent watts would be Vs -£* /. When two wattmeters
are used, as shown in Fig. 54, the power factor of a three-
phase circuit can be determined from the ratio of the read-
ings alone, and ammeter and voltmeter readings are not
necessary. The ratio of the readings is

VS £ / cos (30^ -F 4>) _ cos(30Q-F^);

V3 £ / cos (30° - *) "" cos (30° - 4>)
cos (30° -F<^ ) cos 30° cos 4> - sin 30° sin <l>

cos (30° -<^ ) cos 30° cos <^ -f sin 30° sin 4>
id s
V3

but cos 30° = -^, and sin 30° = i; hence.

cos 4* — T sin 4^ r-

cos (30° -F <l> ) ^ 2^ ^ V3 cos ^ ~ sin <^

cos (30° - * ) V3 ^^ . . 1 ^. . ^lScos4> + sin <l>
— - cos 9 + t sm 9
z

^3 cos ^ ""* sin ^

Now if we take the expression -.- , and sub-

V3 cos ^ -h sin <^

stitute different values for <^, we will get the ratio of the

wattmeter readings corresponding to those values. For

example, if <^ = 60° we have ratio of wattmeter readings

^sxi-4

= -^ = 0. An angle of lag of 60° cor-

V3xi + -| ^
responds to a power factor of .5. For an angle of lag of
*E. J. Berg. Electrical World and Engineer, Vol. XXXIX.

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

30°, power factoi = cos 30° = .866, we have ratio of readings

V3xf + i

1
2

By thus taking different values of the power factor we can
plot a curve, Fig. 57, showing the relation between the ratio
of the wattmeter readings and the power factor of the load.

Example. — The power supplied to a three-phase induction motor is
measured by means of two wattmeters connected as shown in Fig. 54.

Iff

•1

••

'^^

r»

f«

Z'-'

iit

»f

i .» .# .T .€ .S .4 .9 ^ .1

.1 .9 A

m ^

1 .4 .B .« .

^atlo cf Read in
(positive)

r .• .» J

(nggative)

g9~

Fio. 67

The reading of A is 2,000 watts, and that of B 6,000 watts. What is
the power factor of the motor corresponding to this load, and what
is the total power supplied to the motor?

2 000

Solution. —The ratio of the two readings is ^';c7^ = .333 and is

D,UUU

positive, because both readings are positive. Hence, referring to

Fig. 57, we take the ratio .333 on the right of the center line, and find

that the power factor corresponding to this ratio is about .74. The

total power supplied to the motor will be 2,000 + 6.000 = 8,000 watts.

62. Power-Factor Indicators. — The power-factor indi-
cator made by the General Electric Company and used on

Digitized by VjOOQIC

§26 AND MEASUREMENT 73

three-phase circuits is based on the foregoing principle. It
consists of a fixed current coil, connected in series with the
middle line, within which two potential coils are mounted on
a vertical shaft. These coils are connected between the
middle and outside lines. The resultant effort tending to
deflect the shaft will evidently vary with the power factor,
because the phase relation of the currents in the movable
coils to the current in the fixed coil will change with the
power factor and the instrument can be calibrated so that
the pointer attached to the movable coils will indicate the
power factor.

Another type of power-factor indicator that is commonly
used is the same in construction as an indicating wattmeter,

Fig. 68

except that the potential coil is connected in series with an
inductance so that the current in it is 90° behind the current
in the main coils when the power factor is 1. The result is
that with a power factor of 1 there is no deflection of the
pointer because there is no torque action between the two
coils. With a power factor less than 1, lagging current, the
pointer swings in one direction and with a power factor
greater than 1, leading current, the pointer swings in the
other direction. Fig. 58 shows the front of a Wagner power-
factor indicator operating on this principle.

63. Measurement of Power With One Wattmeter.

The power supplied to a balanced three-phase load may be
measured with a single wattmeter, as shown in Fig. 59, by

4&— 82

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

first taking a reading with the potential coil connected at c
and then quickly transferring the connection to c'. The sum
of the two readings will give the power if the power factor
is over .5; if less than .5, the difference in the readings would

ToUa£^

"TRJWW5 —

.2i^y

Fio. 60

be taken. It is necessary, however, to use two wattmeters
unless the load can be kept constant while the connections
are being changed or in case the load is not balanced.

»/Mir

Fio. 60

64. Power Measurement on Monocyclic Circuit.

Fig. 60 shows the connections for a Thomson recording
wattmeter measuring energy supplied to a motor operated
on the monocyclic system. The meter has two coils Ay By

Digitized by VjOOQIC

§26 AND MEASUREMENT 75

which are connected in series with two of the lead wir^s run-
ning to the motor. As shown in the figure, the coils are in
series with the leads C, D. If it is found that the speed of
the meter diminishes when the load on the motor increases,
field coil A should be connected in series with the main E
instead of C

INSTAIiliATION OF RECORDING WATTMETERS
65. liocatlon. — Recording wattmeters should be located
so that they can be easily reached either for the purpose of
taking readings or inspecting them. They are too often
placed in out-of-the-way places where they are very difficult
to get at. ^They should not be placed in a position where
they will be subjected to vibration as, for example, near
a door that is continually being opened and shut. The loca-
tion should be such that the meter will not be exposed to
dampness or chemical fumes of any kind.

CONNECTIONS FOR METERS

66. The method of connecting meters to the circuit varies
with the size and make of the meter. It is impossible to

Fig. 61

^v^ here all the different connections and, moreover, it is
not necessary or desirable to do so, as the makers send

Digitized by VjOOQIC

76 POWER TRANSFORMATION §26

PIO.Q2

Pig. 63

Digitized by VjOOQIC

§26

AND MEASUREMENT

out instructions with the meters, and these instructions are
liable to change with changes in the construction of the
meters. Therefore, only a few of the most common connec-
tions used on direct-current or single-phase alternating-
current circuits will be described.

67. Connections for Thomson Recording Watt-
meter.— Fig. 61 shows the method of connecting a Thomson
recording wattmeter of small capacity on a two-wire circuit.

-To Load

To Trans fc

fa)

-To Transfornner

_L_

To Load — »

Pig. 64

When the meter is of large capacity, only one side of the
circuit is run through it and a small potential wire is run in
from the other side, so as to put the armature across the
circuit. This method of connection is shown in Fig. 62.
Fig. 63 shows a meter connected to a three-wire circuit.

68. Connections for Stanley Induction Wattmeter.

Fig. 64 {a) and {b) shows the methods of connecting a
Stanley wattmeter. The black terminal B on the meter
must always connect to the transformer or other source of

Digitized by VjOOQIC

POWER TRANSFORMATION

§26

E. M. F. The white terminal W connects to the load. It
is necessary to have these connections correct or the meter
will not rotate in the proper direction. The potential wire p
connects from the meter to the wire that does not enter
the meter.

The connections for induction wattmeters are much the
same no matter what the make may be, the current coil or
coils being connected in series with the circuit, and the
potential coil across the circuit. What differences there
may be are due to the manner in which the leads are brought

Poterfflal 6tiufo
Current Stud^

lb Line ^

fUfi ^^

'ToLomt

Pio. 66

out of the meter case. In most cases current transformers
are used in connection with meters on high-tension lines,
the current coils being connected in series with the second-
ary of a current transformer instead of in series with the
main circuit. On high-potential circuits, the potential coils
are supplied from potential transformers that step-down the
voltage applied to the coils. Of course, when current and
potential transformers are used in connection with a meter,
the instrument is always calibrated so that it will take
account of the current or voltage transformations and

Digitized by VjOOQIC

AND MEASUREMENT

indicate the number of watts in the main circuit. Fig. 65
shows the connections for a General Electric induction watt-
meter of the polyphase type used on switchboards. In this
case, potential transformers /, /' are used to step-down the
voltage and current transformers /, P to transform the cur-
rent. The connections shown are such as would be used on
a three-phase circuit or a three- wire, two-phase circuit with
the common return wire in the middle.

TESTING AND ADJUSTING RECORDING
WATTMETERS

69. Recording wattmeters should be checked up occa-
sionally to see if they record correctly. If a rough test
only is required the meter may be loaded with a specified
number of lamps of which the power consumption per lamp
is known; if a more accurate test is desired, the recording

Pio. 66

meter is usually checked by comparing it with a standard
indicating wattmeter.

70. Checking a Thomson Recording Wattmeter.

Figs. 66 and 67 show connections for checking a two-wire
Thomson meter. Either set of connections may be used.
The meter is set to work on a load of lamps, or other con-
venient resistance, the standard direct-reading wattmeter

Digitized by VjOOQIC

80.

POWER TRANSFORMATION

§26

being connected as shown. A chalk mark is made on the
meter disk, so that the revolutions may be easily counted,
and the revolutions are taken for 40 to 60 seconds, the
observer using a stop-watch. Another observer reads the
standard instrument, and the load is kept as nearly constant
as possible throughout the test. The meter watts may then
be calculated from the following formula:

Meter watts =

3,600 y? A'

(1)

where R = number of revolutions in T seconds;

T = time in seconds of R revolutions;

K = constant of meter.
The constant K used in formula 1 was, in the older types
of meter, marked on the dial and was a number by which
the dial reading had to be multiplied to give the true reading

Pio. 67

of the meter. In recent types of Thomson meter, the gears
in the recording train are arranged so that the dial reads
directly and no constant is marked on it except in meters of
large capacity. In recent meters the constant K used in
formula 1 will be found marked on the revolving disk.

The actual watts are obtained from the standard meter;
hence, the percentage by which the meter is correct is found
by dividing the number of watts given by formula 1 by the
number of watts given by the standard meter.

Digitized by VjOOQIC

§26

AND MEASUREMENT

Example. — The disk of a 10-ampere, 100-volt Thomson meter makes
10 revolutions in 60 seconds. The average number of standard watts
as indicated by the standard meter is 303. Find the percentage error
of the recording meter. The constant of the meter is i.

Solution. — From formula 1, we have

3,600 X 10 X i

Meter watts

= 300

m = .99, or 99%. Ans.

The meter is, therefore, 1 per cent, too slow, and the damping mag-
nets should be shifted in a little so that the retarding action on the
disks will not be so great.

71. If a standard wattmeter is not available for testing
purposes, separate ammeters and voltmeters may be used
for direct-current work, but they are not as convenient.

Line

Load

Pig. 08

In Figs. 66 and 67 it will be noticed that the energy con-
sumed by the potential circuit of either meter is not
measured by the other; that is, the ciurent in the armature
of the Thomson meter does not pass through the fields of
the standard meter, neither does the current in the shunt
of the standard pass through the field cofls of the Thomson
meter.

To test a Thomson meter used on a three-wire circuit
(110-220 volts), the connections may be made as shown in
Fig. 68. The potential circuits are wound for 110 volts.
The field coils can, therefore, be connected in series, and the
standard meter connected as shown. In formula 1, however,

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POWER TRANSFORMATION

§26

K should be taken as one-half the constant marked on the
dial or disk. Aside from this, the meter can be tested in
the same manner as a two-wire meter.

72. Checklngr a Stanley Wattmeter. — Fig. 69 shows
the connections for checking a Stanley wattmeter and the
connections for testing any two-wire induction wattmeter

Pio. 69

would be very similar. A is the recording wattmeter and B
the standard instrument. With the Stanley meter the watts
are given by the following formula:

\mRK

Meter watts =

(2)

where only R — number of revolutions in T seconds;
T = time in seconds for R revolutions;
A^ = a constant marked on the meter case.
This formula applies also to the Fort Wajme induction
meters, the values of K being given for different sizes of
meters, in a table furnished by the manufacturers.

READING RECORDING WATTMETERS

73. The dials of most wattmeters record either watt-hours
or kilowatt-hours. In some cases, as with the earlier types of
Thomson meter, the reading taken from the meter dials must
be multiplied by a constant in order to obtain the watt-hom-s.
This constant is usually marked on the dial. However, the
general practice now is to make the dials of .meters direct
reading except in the case of meters of large capacity. If no
constant is marked on the dial it can be assumed that the
meter is direct reading.

Digitized by VjOOQIC

f^QOO,O0O

1 ir^

/,coo

fO^OOf^OOO

iO^ooe^ooo

FiO. 70

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84 POWER TRANSFORMATION §26

74. Readings Tbomson Meter. — The Thomson meter
has five dials. The lowest reading pointer is the one to the
extreme right (facing the meter); it is marked 1,000, which
means that one complete revolution of the hand indicates
1,000 watt-hours, and that each division therefore represents
100 watt-hours. The next one to the left is 10,000 to a
revolution, or 1,000 for a division, and so on. Fig. 70 shows
six different readings, by studying which the student should
be able to take readings from any meter.

Beginning at the left, number the pointers 1, 2, 3» 4, and 5.
Then, in /, Fig. 70, pointer 5 is on 2 and is read 200.
Pointer 4 is two-tenths of the way between 8 and 9 and is
read 8,000. Pointer 3 is read 10,000. Pointer 2 has not
jfone through its first division; likewise pointer 1. The state-
ment is then 18,200.

The statement of // is 5,718,900 (not 5,719,900, as it fre-
quently would be read) . Pointer 4 should not be read 9 until
pointer 5 has completed its revolution and is again at 0.

The statement of ///is 99,800 (not 109,800), because the
100,000 mark will not be reached until pointer 5 has passed
from 8 to Oy when 4 and 3 will be at 0, pointer 2 at 1, and
pointer 1 just past the zero mark.

The statement of /F is 9,990,800. Pointer 1 is slightly
misplaced. Otherwise, the reasons given above will apply
to this statement.

The statement of V is 8,619,900. Pointer 2 is misplaced.
It should be two-tenths of the way between 6 and 7 instead
of nearly over 6y as shown.

The statement of F/ is 834,200. Pointer 4 is misplaced.
It should be two-tenths to the right of 4 instead of to the left
of 5. These misplaced hands are frequently met with in
practice and are generally caused by a knock in removing the
cover, or, perhaps, they are a little eccentric.

Rule. — To ascertain the number of watt-hours that has been
used by a consumer from o?te date to another, subtract the earlier
statement from the latter and multiply by the constant of the
meter y if one is marked on the dial. In case no constant is

Digitized by VjOOQIC

§26 AND MEASUREMENT 85

marked on the meter ^ the constant is 1, and the readings are
taken as given by the dial,

ExAMPLB. — ^An electric company supplies power to operate a motor
for one of its customers. The rate charged is 5 cents per kilowatt-
hour. The reading of the meter on January 30 is 8,619,900, and on
February 28. it is 9,990,800. The constant of the meter is 2. What
should be the amount of the bill for the month?

Solution. — The number of watt-hours supplied between Jan. 30
and Feb. 28 » (9,990,800-8.619,900) X2 = 2.741,800.

2,741,800 watt-hours = 2,741.8 K. W.-hours, which at 5 cents per
K. W.-hour would amount to 2,741.8 X .05 = $137.09. Ans.

SPECIAIi METERS

75. The Two-Rate Meter. — Most electric-light stations
have their period of heaviest load for a few hours only in
the evening. During the daytime the plant is lightly loaded,
and a large part of the machinery is standing idle. In order

Pio. 71

to obtain a day load and thus work the plant to best advan-
tage, some companies supply power during the daytime at
specially low rates in order to induce customers to use
electric motors. For measuring the power supplied to such

Digitized by VjOOQIC

86 POWER TRANSFORMATION §26

customers, two-rate meters are sometimes used. A trwo-rate
meter is one that records the power during certain hours of
the day at a rate different from that at other hours. One
of the earlier types made by the General Electric Company
was a regular Thomson recording meter provided with two
dials and recording trains, which were arranged so that
a self-winding clock would throw either one or the other
into gear with the meter shaft at the proper time. The
energy recorded on the two dials was then charged for at
different rates.

In the later t3rpe of General Electric two-rate meter an
ordinary Thomson meter A, Fig. 71, with a single dial is
used. Connected to the potential circuit of ^ is a self-
winding clock mechanism contained in the case B, The
case also contains a resistance, which, during certain hours,
is inserted in series with the armature of the wattmeter,
thus making the meter run at a reduced speed during those
hours. The two-rate attachment, therefore, makes the
meter run slow during certain hours, which is equivalent
to charging for the power at a low rate during those hours.

76. Maximam- Demand Meter. — The maximum
amount of current that the various customers consume
determines in large measure the capacity of the station equip-
ment. Some customers might use large currents for short
intervals only, but the plant must be capable of handling
these large currents; in some cases, therefore, the maximum
demand for current is taken into account in making up the
bill; for example, all current over a certain amount is charged
for at a higher rate. One style of instrument used for indi-
cating the maximum current used above a certain amount is
the Wright maximum-demand meter^ shown in Fig. 72. It
consists of a U-shaped tube, hidden partly by the scale in the
figure, which has bulbs Ay B on either end; a branch tube C
is attached near B and carried down over the scale. The
lower end of tube C is closed. The current flows from D to
E through the resistance strip F coiled around bulb A. The
tube is partially filled with liquid, which remains in it as long

Digitized by VjOOQIC

§26 AND MEASUREMENT 87

as the current does not exceed a certain amount. If , how-
ever, the current exceeds the allowable amount, the expan-
sion of the air in A due to the heating of strip F will force
liquid into the tube C Any increase in the current will force

Fio. 72

over more liquid, and from the height of the column of
liquid in tube C the charge can be estimated. The U-shaped
tube is mounted on an arm that can be swimg up after the
reading has been taken, thus emptying tube C into the
U-shaped tube.

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INDEX

NoTB.— AH items in this index refer first to the section and then to the pasre of the
section. Thus, "Bus-bars 25 19" means
section 25.

that bus-bars will be found on pa^e 19 of

Paze

Adjustinc: and testinjr recordinjr

wattmeters 26

Air firap. Density in 20

*• •• Lensrth of 22

" and bore of

poles .... 21
'* " of induction motor,

Lensrth of 22

All-day efficiency of transformer . 22
Altematinff-current apparatus. De-

sismof 20

*' current apparatus. De-

sisniof 21

** current apparatus. De-

sifimof 22

** current. Arresters for 25
** current. Line calcu-
lations for 28

** current. Power trans-
mission by 23

** current switchboards . 25
" current systems ... 23
current systems. Fre-
quency in 28

Alternator, armature. Heatinsr of . 20
" Armature windins: for

three-phase ..... 21
** Armature windins: for

two-phase 21

*' armatures. Peripheral

speed of 20

Desism of 100-kilowatt
sinsrle-phase .... 21
Alternators. Combined runninsr of 23

Design of 20

" Electrical c o n n e c -

tions for 21

Huntinsrof 23

inparaUel 23

" ** parallel. Com-
pound-wound . . 23
•• series 28

11
6

10
26
47
58

Sec. Page Sec.

Aluminum. Comparison of prop-

79 erties of copper and 24

48 " conductors 24

88 *' line wire. Resistance.

tensile strength, and

28 weififhtof 24

** wire. Deflections and

87 tensions for . 24

28 ** " Resistances of

pure 24

1 " ** Stringrinsr.'. . . 24

Ammeter connections 26

1 Apparatus and line tests 24

" for controllin&r incom-
1 ins: current ... 26 20

54 ** " controllinsr o u t -

sroin&r current . . 26 40

80 " " transformlnsr cur-
rent 26 26

23 ArcinfiT, Suppression of ^ 49

76 Area of B. & S. wires, Sectional . 23 10

39 Armature conductors 20 38

core 22 56

41 •• *• Density in .... 20 47

4 " ** Desigm of 21 4

disks 20 31

15 " inductance. Calculation

of 20 18

18 " insulation 20 42

** losses. Calculation of . 21 10
20 " Mechanical construc-
tion of 22 56

1 '* or rotor 22 30

58 " Radiatins: surface of . . 20 10

1 ** reaction 20 11

self-induction 20 15

57 " slots. Insulation of ... 20 <A
71 *• spiders 20 34

58 *• teeth. Density in .... 20 46
windings 20 21

76 " windinff for three-phase

58 alternator 21 15

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INDEX

Sec. Paxe
Armature windini; for two - phase

alternator 21 13

windin&rs. Polyphase . . 20 27

Armatures, Completed 21 19

Heatins of alternator 20 4
" Peripheral speed and

diameter of 22 42

Peripheral speed of

alternator 20 20

Arrangrcment of windings 20 29

Arrester for altematinsr current.

General Electric . 25 66
" altematinsr current,

Westinsfhouse . . 25 bi
" " high-tension lines,

Westinsrhouse . . 25 56

Garton 25 51

General Electric .... 25 63

Simple lisrhtninsr .... 25 48

Westinghouse 25 62

Westinghouse low-
equivalent 25 57

Arresters for alternating current 25 54

** ** direct current ... 25 61
Ground connections for

lightning 25 50

" Installation of 25 60

Location of 26 60

Bare and insulated wires 3 1

** copper wire. Dimensions,

weights, etc. of 24 4

Bed. frame, and field. Construction

of 21 43

Bedplate and field frame of induc-
tion motors 22 67

Bituminized-fibcr conduit 24 38

Bore of poles and length of air gap 21 28

Box. Four-way 24 47

Boxes. Junction 24 46

Service 24 50

Brush-holder studs 21 51

holders and brushes .... 21 60

Bus-bars 25 19

** Carrying capacity of . . 25 20
** " Mounting for high-
tension 25 21

Cable. Drawing in 24 43

joint, High-tension 24 61

Cables. Distribution of. for man-
holes 24 45

Joining 24 61

Main and equalizer .... 23 66

Sec.
Calculation of armature induct-
ance 20

" armature losses . 21
** " primary and sec-
ondary turns . 22
* " separately-ex-
cited winding . 21
Calculations for alternating cur-
rent. Line 23

" for two-wire system 23

** Formulas for line . . 23

Line .• . 23

Capacity of transformers for three-
phase induction motors 26
" of transformers on two-
and three-phase sys-
tems 26

** of underground tubes.

Carrying 24

Carrying capacity of bus-bars . . 25
" " underground

tubes ... 24

Cement-lined pipe conduit 24

Checking a Stanley wattmeter . . 26
** of Thomson recording

wattmeter 26

Choke, or reactance, coils 25

Circuit-breaker and slide switch,

Stanley 25

Cutter ...... 25

" " Cutter laminated-
type 25

•* " General Electric

MK 25

breakers 25

** " and fuses .... 25

General Electric 26
Circuits, Induction wattmeter for

unbalanced i>olyphase 26
** Measurement of power

on three-phase 26

" Measurement of power

on two-phase 26

Transformers on single-
phase 26

** Transformers on three-
phase 26

Transformers on two-

phaso 26

" Use of two wattmeters

on three-phase 26

Coils and core. Arrangement of . 22
** Arrangement of primary and

secondary 22

Pu£e

18
10

67
20

57
33

79
49

8
35

5
31

47
27
32

66
6

Digitized by VjOOQIC

INDEX

Sfc. Pane

Cofls. Pield-masmet 21 25

Insulation of armature ... 20 42

" field 21 27

•• Klckinar 25 49

" Loss in field 21 42

" Reactance, or choke .... 25 49
** Windinsr and insulation of . 22 8
Collector n'nsrs and rectifier .... 21 45
Combined operation of direct-cur-
rent dynamos .... 23 45
** runnins: of alternators . 23 58

CompensatinsT voltmeter 25 28

Compensator, Mershon 25 25

Completed armatures 21 19

Compound machines in parallel . 23 50
" machines in parallel

with shunt machines 23 58

** or series-field, windin&r 21 38
** wound alternators in

parallel 23 76

Conductor and core. Dimensions of 21 8

Size of primary .... 22 42

Conductors. Aluminum 24 6

and core. Rotor ... 22 50

" Armature 20 88

Copper 24 1

Cost of 23 43

** Dimensions of .... 22 13

Line 24 1

" of low resistance. Lo-

cating srrounds and

crosses on 24 56

Conduit. Bituminized-fiber .... 24 38

Cement-lined pipe .... 24 83

Creosoted-wood 24 82

'* Pump-loar 24 33

Vitrified-clay or terra-
cotta 24 34

Conduits 24 32

Connection, Equalizer 26 47

" Bqualizinsr 23 47

Connections. Ammeter 26 47

Field windin&r and . . 22 55
" for alternators. Elec-
trical 21 57

•* for meters 26 75

'* for six-phase rotary

converters .... 26 82
** for Stanley induction

wattmeter 26 77

** for substations ... 26 44

** for synchronizins: . . 26 46
•* for Thomson record-

infir wattmeter ... 26 77

Shunt-field 26 48

Voltmeter 25 22

Sec.

Connections, Voltmeter 26

Construction. Line 24

" of armature. Me-

chanical ... 22
** " collector rines

and rectifier . 21
" ** switchboards . 25

** Overhead 24

line ... 24

*' of shafts 22

** ** transformers . . 22

" Undersrround .... 24

line. . 24

Continuity tests 24

Converter, Direct-current 23

Converters. Connections for six-
phase rotary .... 26
" Methods of startini;

rotary 26

Rotary 26

** Synchronizins: rotary 26

VoUasre regulation of

rotary 26

Copper and aluminum. Compari-
son of properties of . . 24

** conductors 24

*• loss 22

** studs. Current densities of 25

** wire 24

" " Dimensions, weififhts.

etc. of bare 24

Core and coils. Arran&rement of . . 22
*• '* conductor. Dimensions

of 21

•• " •• Rotor .... 22

*' Armature 22

*• Construction and arrange-
ment of transformer ... 22

** Density in armature 20

** Desism of armature 21

" Dimensions of 22

" losses 20

'* " and masrnetic densities 22

** Masmetic density in 22

** type transformers on three-
wire system 26

** volume. Determination of . . 22

Cores. Transformer 22

Cost of conductors 23

Creosoted-wood conduit 24

Cross-arras 24

section of lines. Estimation

Crosses and srrounds on conduct-
ors of low resistance. Loca-
ting 24

Page
46

45
71

1
14
56
27

1
32

85
81
39

1
1
3

3
50
56

12
7

31
5

11
4

43
32
16

23 81

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Xll

INDEX

Sec. Page

Crosses or srrotinds. Testing for . 24 59

Tests for . . 24 62

CR rcfirtilator 25 44

CuiTcnt densities for copper studs 25 8
densities per square inch

in induction motors . . 22 89

detector sralvanometer . . 24 60

Half of main 26 45

in lines. Estimation of . . 23 82

•* primary. Full-load ... 22 41
" stator and rotor of in-
duction motors.

Volume of 22 89

Mafirnetic 22 24

*' Power transmission by

direct 28 2

Curve. Efficiency 22 21

Ctitter circuit-breaker 25 35

laminated-type circuit-
breaker 53 25

Dampinsr devices. Use of 23 78

Deflections and tensions for alumi-
num wire 24 27

Densities, Mafimetic 20 46

Density in air srap 20 48

** armature core 20 47

teeth 20 46

" core. Magnetic 22 5

** of masmctism in rotor

teeth . 22 89
" " " in stator

teeth . 22 39
Deslgm of alternatinsr -current

apparatus 20 1

*' " alternatinsr -current

apparatus 21 1

" alternatinsr -current

apparatus 22 1

** '* armature core .... 21 4

•• field 21 28

** 8-kilowatt transformer 22 10

" lO-horscpower motor . 22 40
" 100 -kilowatt sinsrle-

phase alternator . . 21 1

" masmets 21 20

Detectors, Electrostatic srround . 25 39

Ground 25 86

Determination of core volume . . 22 11
Diameter and speed of armature.

Peripheral 22 42

Dimensions and resistance of iron

wire 24 12

** of conductors .... 22 13

•• •* conductor and core 21 8

See, Page

Dimensions of core 22 12

" knife switches ... 25 4

•' poles 24 15

*' weisrbts, etc. of bare

copper wire .... 24 4

Direct-current, Arresters for ... 25 51

" ** converter 23 22

dynamos 23 45

" " Power transmis-
sion by 23 2

'* *• switchboards ... 25 73

** '* systems 23 36

Disks, Armature 20 81

Distribution of cables for man-
holes 24 45

Dobrowolsky three-wire system . 23 20
Double-current srenerator installa-
tion 25 86

Drawing fn cable 24 43

Drop, Estimation of 23 33

Dynamos and motors for direct-
current power trans-
mission 23 2

" Direct-current 23 45

" in parallel. Series ... 28 47

Shunt ... 23 48

** series. Operation of . 23 45

Eddy-current loss 20 9

Edison three-wire system 23 15

** undersrround-tube system . 24 58

Efficiency curve 22 21

Pull-load 22 37

** of transformer 22 19

vAU-day. 22 28

Blectn'c transmission 23 1

Electrical connections for alterna-
tors 21 57

Electrostatic srround detectors . . 25 39

Equaliser cables. Main and .... 23 56

connection 26 47

Equalizing connection 23 47

Equipment. Substation 26 18

Estimation of cross-section of lines 23 31

*• current in Ikies ... 23 32

** " drop 23 83

Faults, Testing Hues for 24 58

Feed-wire, Standard weather-proof 24 8

Feeder panels 25 72

Field coils. Insulation ef 21 27

'* Loss in 21 42

•• Desisrnof 21 28

'* frame and bed. Construction

of 21 a

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INDEX

Xlll

Sec. Page
Field frame and bedplate of induc-
tion motors 22 57

" masmet coils 21 25

" macrnets. Desism of 21 20

** orstator 22 80

" rheostat. General Electric . . 25 67

" rheostats 25 65

switches 25 67

" wiudinsr and connections . . 22 56

Fields, Revolvlnir 21 23

Flux in poles. Magnetic 22 45

Formulas for determininsr resist-
ance of wire 24 11

for line calculations . . 23 81

Fort Wayne induction wattmeter . 26 57

Four-way box 24 47

" wire system. Two-phase . . 26 59
Frame, field, and bed. Construc-
tion of 21 43

Frequency in alternating-current

systems 28 41

Full-load current in primary ... 22 41

*' " eflSciency 22 87

•• power factor 22 88

•• " Slip at 22 40

Fuses 26 28

and ci^uit-breakers .... 25 27

•* Primary 26 2

Galvanometer. Current-detector . 24 60

Garton arrester 25 61

Gauares. Wire 24 8

General Electric arrester 25 58

Electric arrester for alter-

natinsr current 25 56

Electric circuit-breakers . 25 32
Electric field rheostat . . 25 67
Electric oil switches ... 25 11
Electric MK circuit-
breaker 25 88

Generator installation. Example

of double-current . . .25 86

panel. Sinsrle-phase . . 25 76

panels 26 72

German-silver wire 24 12

Ground connections for liffhtninsr

arresters! 25 50

" detectors 25 86

Electrostatic . . 25 39
** and crosses on conductors
of low resistance. Lo-
cating 24 66

•• or crosses. Testinsr for . . 24 69

Tests for ... 24 62

Half of main current

Heat losses

Heatinsr of alternator armatures .

** transformers

Hififh pressure. Use of

** switchboards . . .
tension bus-bars, Mountinsr

for

** cable joint

'* lines. Leakasre on .

*• " Westinffhouse

arrester for

" switches

" systems. Static

effect on

Huntinsr of alternators

Hysteresis loss

Indicatinsr wattmeters

Indicators, Power-factor

Inductance. Calculation of arma-
ture

Induction motor. Lcnsrth of air sap

" windings ....

motors

motors. Capacity of
transformers for three-
phase

motors. Current den-
sities per square inch

motors. Field frame and

bedplate for

motors. Gereral data on
** motors. Limitation of

output of

motors. Number of poles

*• motors. Peripheral

speeds of

motors. Power factor of
* motors. Primary wind-
ins: of

** motors. Secondary

winding: of

" motors. Volume of cur-
rent in stator and

rotor

wattmeter. Connections

for Stanley

wattmeter for unbal-
anced polyphase cir-
cuits

wattmeter. Port Wayne

Sec.

Page

20 18

22 37
22 33
22 80

26 17

22 39

Digitized by VjOOQIC

»▼

INDEX

Sec. Page

Induction wftttmeter. Stanley ... 26 58

wattmeters 26 54

Iron losses 22 1

" wire 24 11

/«^loss 22 1

** ** loss and output.^ Relation

between 20 6

Installation of arresters 25 00

** recordinsr watt-
meters 26 76

Insulated wires. Bare and 24 1

Insulatinfif armature slots 20 48

Insulation and winding of coils . . 22 8

of armature coils ... 20 42

** field coils 21 27

** resistance. Tests for . . 24 61

Insulators 24 20

Types of 24 21

Interrupter. Static 25 63

Joining cables 24 51

Joint. Hlsrh-tension cable 24 51

Junction boxes 24 46

Kickinarcofls 26 49

Knife switches. Dimensions of . . 25 4

Laminated-type circuit -breaker.

Cutter 26 85

Lamps, Synchronizinsr 28 60

Leakage on hitrh-tension lines ... 24 81

Lensrth of air srap 22 88

Lisfhtinsr or power switchboard . . 25 74
Lisrhtningr arresters. Ground con-
nections for 25 60

arrester. Simple .... 25 48

Protection from .... 25 47

Limitation of output 20 2

of output of induction

motors 22 81

Lincoln synchronizer 28 65

Line and apparatus tests 24 58

•• calculations 28 7

" *• for alternatinsr

current .... 23 80

•* " Formulas for . . 28 31

•* conductors 24 1

•* construction 24 1

•• " Overhead .... 24 14
Undergrround . . 24 32
•* drop. Lost power and .... 28 4
** protection by continuous dis-
charge . 25 61

Sec. Page

Line protection from static charEes 25 62
" wire. Resistance, tensile
strensrth, and weight of

aluminum 24 9

Lines. Estimation of cross-section

of 23 81

** " ** current in . . 23 32

** Leakage on hifrh-tension . . 24 31

" Testing, for faults 24 58

** Transportationlof transm is-

sion 24 28

Locating a cross by the Varley

loop method 24 65

** a partial ground without

an available STOod wire 24 64
** grounds and crosses on
conductors of low re-
sistance 24 66

Location of arresters 25 60

Long shunt 28 51

Loop test. Varley 24 62

Loss, Copper 32 1

Eddy-current 20 9

Hysteresis 20 7

" nR 22 1

*' in field coils 21 42

Losses, Calculation of armature . 21 10

Core 20 7

Heat 22 52

Iron 22 1

Lost power and line drop 23 4

Low-equivalent arrester. Westing-
house 25 57

** tension switches 25 2

Machines in parallel. Compound . 28 SO

Magnetic current 22 24

** densities 20 46

*• " and primary

core losses . 22 31
" " and secondary

core losses . 22 32

" density in core 22 5

" flux in poles 22 45

** ** through pole pieces

and yoke 21 80

Magnetism in rotor teeth. Density

of 22 39

" stator teeth. Density

of 22 39

Magneto testing set 24 58

Magnets, Design of field 21 20

Main and equaliser cables .... 23 56

Main current. Half of . / 26 45

Manholes 24 88

Digitized by VjOOQIC

INDEX

Sec. Paze

Maximum-demand meter 26 86

Measurement and transformation

of power 26 1

•• of power factor . . 26 71

•* of power on poly-

phase circuits . . 26 58
*• of power on three-

phase circuits . . 26 63
" of power on two-

phase circuits . . 26 59
** of power with one

wattmeter .... 26 73

Mechanical construction 21 43

Mershon compensator 25 25

Meter, Maximum-demand .... 26 86

. '* Reading Thomson 26 84

Two-rate 26 85

Meters, Connections for 26 75

Special 26 86

MK circuit-breaker. General Elec-
tric 25 38

Motor, Desism of 10-horsepower . 22 40
'* Lensrth of air srap of induc-
tion 22 87

Motors and dynamos for power
transmission by direct-
current 23 2

Capacity of transformers
for three-phase induction 26 17
•• Current densities per

square inch in induction 22 39
** Field frame and bedplate

for induction 22 57

" General data on induction /22 37

Induction "22 80 -^

** Limitation of output of in- -

duction 22 31

•* Number of poles of induc-
tion 22 87

** Peripheral speeds of in-
duction 22 87

** Power factor of induction 22 86
'* Primary windin&r of induc-
tion 22 33

" Secondary winding: of in-
duction 22 85

" Volume of current in stator

and rotor of induction . 22 39
Mountinsr for bisrh-tension bus-bars 25 21

Oil switch of larsre capacity .... 25 15

*• switches. General Electric . . 25 11

Stanley 25 19

Operation of direct-current dyna-
mos. Combined 23 45

Ste. Paze
Operation of direct-current dyna-
mos in parallel . . 28 45
" " dynamos in series . 28 45

Output, Limitation of 20 2

of induction motors. Limi-
tation of 22 31

Overhead construction 24 1

line construction .... 24 14

Panel, Sinsrle-phase generator . . 25 76

Panels, Feeder 25 72

** Generator 25 72

Parallel operation. Features con-
nected with 23 69

Peripheral speed and diameter of

armature ... 22 42
** " of alternator

armatures : . 20 20
" speeds of induction

motors 22 87

Phase-chanarinsr transformers . . 26 13

Pins 24 19

Pipe conduit. Cement-lined .... 24 33

Plus: switch, Stanley 25 6

Plunder switch, Westinsrhouse . . 25 7

Poles. , 24 14

'* Bore of, and lenrth of air

arap 21 28

'* Dimensions of 24 15

'* Macmetlc flux in 22 45

" of induction motors. Number

of 22 37

*• Selection of 24 14

** Sizes of 24 14

" Spacing: of 24 15

Polyphase armature windings . . 20 27
** circuits. Measurement

of power on 26 53

*• transformers 26 29

Potential regulators 25 42

Power factor. Full-load 22 38

•• indicators 26 72

** ** of induction motors 22 36
** measurement. Instruments

used for 26 53

" measurement on mono-
cyclic circuit 26 74

** or lighting switchboard . . 25 74
" on three-phase circuits.

Measurement of 26 63

** transformation and

measurement 26 1

** transmission by alternating

current 23 23

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XVI

INDEX

Sec. Page
Power transmission by direct cur-
rent .... 23 2
" ** system. Sim-
ple 23 3

Three-phase. 28 28

*• ** Two-phase . 28 26

Pressure wires 24 47

26 28

Primary and secondary coils ... 22 16

turns. . . 22 16

conductor 22 42

" core losses and masmetic

densities 22 81

Pull-load current in ... 22 41

fuses 26 2

windinsr 22 83

22 43

** ** of induction

motors .... 22 88
Protection from tisrhtninsr and

static charsres 26 47

Pulleys 21 65

Pump-losr conduit 24 S3

Radiatinsr surface of armature . . 20 10

Railway switchboard 26 73

Reactance, or choke-coils 26 49

Reaction, Armature 20 11

Reading: recordins: wattmeters . . 26 82

Thomson meter 26 84

Recordins: wattmeter. Checkinsr of

Thomson 26 79

** wattmeter. Connections

for Thomson .... 26 '77

** wattmeter, Thomson . 25 69

** . 26 54

wattmeters 25 69

.• 26 54

** " Installation

of .... 26 75

Readingr . . 26 82
*• ** Testinsrand

adjusting 26 79

Rectifier and collector ringrs ... 21 45

Resrulation of transformers .... 22 25
** " rotary converters,

Voltasre 26 32

Voltage 20 2

Resrulator, CR 25 44

Stillwcll 25 43

RcfiTulators. Potential 25 42

Relation between I^ R loss and

output 20 6

Relay, Reverse-current 26 28

Sec. Pia£e

Relay. Time-limit 26 20

** Westinfifhouse time-limit . 26 28
Resistance and dimensions of iron

wire 24 12

" of wire. Formulas for

determining 24 11

Test for insulation . . 24 61

Reverse-current relay 26 28

Revolvinsr fields 21 28

Rheostat. General Electric field . . 26 67

Rheostats. Field 26 65

Rotary converters 26 81

•* " Connections for

six-phase . . 26 82
" *• Methodsof

startins: ... 26 35
** ** Synchronizins: . 26 39
*• •• Voltasre regula-
tion of .... 26 32

Rotor or armature 22 30

conductors and core .... 22 50
*' teeth. Density of magnetism

in 22 89

Rule for determininsT size of wire

for a ffiven transmission .... 28 9

S
Scott two-phase, three-phase trans-
former 26 15

Secondary coils. Arranfirement of

primary and .... 22 16
** core losses and masr-

netic densities .... 22 82

winding 22 60

** windinsr of induction

motors 22 35

Sectional area of B. A S. wires . . 23 - 10

Selection of a system 23 36

Se.lf-induction. Armature 20 15

Separately-excited windinsr. Calcu-
lation of .21 34

Series dynamos in parallel .... 23 47

** field, or compound, windinsr 21 38

** Operation of dynamos in . 23 45

Service boxes 24 60

Shafts 21 54

Construction of 22 56

Short shunt 23 51

Shunt dynamos in parallel .... 23 48

•* field connections 26 48

*' Lons: 23 51

" machines. Compound ma-
chines in parallel with . . 23 58

" Short 23 51

Simple lififhtnins arrester 26 48

** power transmission system 28 S

Digitized by VjOOQIC

INDEX

xvii

Sec.
Slngrle-phase alternator. Desiffn of

lOO-kflowatt .... 21
*• " circuits. Transform-
ers on 26

concentrated wind-
ing 20

" srenerator panel ... 25
" transmission .... 23
** wattmeter on two-phase cir-
cuit 26

wattmeter with resistance . 26
Siz-p)iase rotary converters. Con-
nections for 26

Size of primary conductor .... 22
**. " wire for a iriven transmis-
sion 23

Sizes of poles 21

Slide switch and circuit-breaker.

Stanley 25

Slip at full load. Table of 22

Slots. Insulation of armature ... 20

Spacing of poles 21

Speed and diameter of armature.

Peripheral 22

" of alternator armatures.

Peripheral 20

Spiders. Armature 20

Splicinfi: and tyinsr 24

Stab switch. Stanley 25

Stanley induction wattmeter ... 26
'* induction wattmeter. Con-
nections for . . - 26

" oil switches 25

•• plus: switch 26

" slide switch and circuit-
breaker 26

stab switch 25

*• wattmeter. Checkins: a . . 26
Startinsr rotary converters.

Methods of 26

Static charsres. Line protection

from 25

** effect on hisrh-tension sys-
tems 25

•* interrupter 25

Stator and rotor of induction

motors 22

or field 22

** teeth, Density of macmet-

ism in 22

Steel wire. Resistance of 24

Stmwcll resrulator 25

Strinsrinsr aluminum wire 24

Studs. Brush-holder 21

Substation equipment 26

** transformers 26

Page
1

76
21

62
65

32
42

9
14

8
40
43
15

20
34
23
10
58

77
19
6

10
82

62
63

11
43
26
51
18
26

Sec. Page

Substations. Connections for ... 26 44
" Location and greneral

arransrement of . . 26 40

Surface of armature. Radiatinsr . . 20 10

Switch of larare capacity. Oil ... 25 15

Stanley plug: 25 6

stab . . 25 10

Wcstinsrhouse pluncer ... 25 7
Switchboard and switchboard ap-
pliances 25 1

appliances 25 1

Power or liffhtint . . 25 74

Railway 25 73

Switchboards 25 71

Altematins:- current 25 76

** Direct-current ... 25 73

•* for parallel runninsr 25 79
•• General arransre-
ment of hiffh-pres-

sure 25 81

•• General construc-
tion of 25 71

Switches 25 1

*' breakinsT arc in a confined

space . . 25 7

** " " in open air 25 5

" under oil. . 25 10

Dimensions of knife ... 25 4

Field 25 67

General Electric oil ... 25 11

Hisrh-tension 25 6

Low-tension 25 2

Stanley oil 25 19

Synchronism 23 59

Synchronizer. Lincoln 23 65

Synchronizing 23 60

** Connections for . . 26 46

lamps 23 60

•• rotary converters . 26 39
** two-phase and
three-phase ma-
chines 23 60

** Use of voltmeter for 23 62

System. Dobrowolsky three-wire . 23 20

Edison three-wire .... 23 15

" Selection of a 23 36

Systems. Altematinsr-current ... 23 39

Direct-current 23 36

Special three-wire .... 23 19

Table of approximate weisfhts of

weather-proof wire . . 24 6
capacity of transformers
for three-phase induc-
tion motors 26 17

Digitized by VjOOQIC

xyin

INDEX

Sec.
Table of carrylnsr capacity of

undersTOund tubes . . 24
" comparison of properties
of copper and alumi-
num 24

" current-densities for

copper studs 25

" deflections and tensions

for aluminum wire . . 24
" density of masmetism in

rotor teeth 22

" density of masmetism in

stator teeth 22

** dimensions and resist-
ance of iron wire ... 24
" dimensions of knife

switches 25

** dimensions of poles ... 24
** dimensions, weififhts, etc.

of bare copper wire . . 24
" full-load efficiency ... 22
" power factor . . 22
" German silver wire ... 24
" lensrth of air firap .... 22
" resistance of pure alumi-
num wire 24

" resistance, tensile
strenfiTth, and" weight of
aluminum line wire . . 24
" sectional area of B. A S.

wires 23

•* slip at full load 22

'* standard weather-proof

feed-wire 24

*' valves of coefficient Af . 28
" volume o f current i n
stator and rotor of
induction motorf ... 22
Teeth, Density in armature .... 20
Tensions and deflections for alumi-
num wire 24

Terra-cotta or vitrified-clay con-
duit 24

Test for grounds or crosses ... 24
" insulation resistance ... 24

'* Varley loop 24

Tcstintr and adjustinsr recordinsr

wattmeters 26

for crosses or firrounds . . 24

lines for faults 24

" set, Masmeto 24

Tests. Continuity 24

Line and apparatus .... 24

Thomson meter, Readinsr .... 26

recordinsr wattmeter . . 25

" . . 26

'axe

Sec.

Paze

Thomson recordinsr wattmeter.

Checkinsr of

recordinsr wattmeter.

Connections for ...

Three-phase alternator. Armature

winding: for . . . .

** ** circuits. Measure-

ment of power on

circuits. Trans-

formers on ... .

circuits. Use of two

wattmeters on . .

" *• power transmission .
** wire system. Core-type

transformers on . .

*' system, Dobrowolsky

*' system, Edison . . .

" 220-volt system ....

•• " 550-volt system ....

*• system.Transformers

•* *• systems. Special . . .

Time-limit relay

" Westinifhouse . .
Transformation and measurement

of power

Transformer, All-day efficiency of

core. Construction

and arrangement

cores

Desism of 8-kiIowatt

BfiBciency of ... .

Scott two-phaie.

three-phase . . .

Transformers

*• and transformer

connections . . .

Construction of . .
for three-phase in-

duction motors.

Capacity of . . .

Heatinsr of

In parallel

on sinsrle-phase cir-

cuits

on three-phase cir-

cuits

on three-wire

system

on two- and three-

phase systems.

Capacity of . . .

on two-phase cir-

cuits

Phase-chansrincr . .

Digitized by VjOOQIC

INDEX

xix

See.

Transformers. Polyphase 26

ReiTuIation of ... 22

Substation 26

Use of, to raise volt-
age 25

Transforming current. Apparatus

for 26

Transmission by altematinsr cur-
rent. Power ... 23
by direct current.

Power 28

Electric 23

lines. Transmission

of 24

Sinffle-phase .... 23
system. Simple

power 23

Three-phase power 23
Two-phase power . 23
Transportation o f transmission

lines 24

Tubes. Carrying capacity of under-
ground 24

Turns. Calculation of primary and

secondary 22

Two-phase alternators. Armature

windimr for 21

" and three - phase systems.
Capacity of transformers

on 26

** phase-circuit. Use of a single

wattmeter on a ... 26
** " circuits. Measurement

of power on 26

" ** circuits. Transformers

on 26

•* ** four-wire system ... 26
" " power transmission . . 23
** " three-phase trans-
former. Scott .... 26

*' rate meter 26

** wattmeters on three-phase

circuits. Use of 26

" wire system. Calculations for 23

•* 220-volt system 23

Tyinsr and splicincr 24

Unbalanced polyphase circuits.

Induction wattmeter for 26

Undersrround construction .... 24
line construction . . 24
tube system. Edison 24
** tut>es, Canyinsr ca-
pacity of 24

Pare V Sec. Page

29 Values of coefficient M 23 34

25 Varley loop method. Locating a

26 cross by 24 65

•• test 24 62

43 Vitrified-clay. or terra-cotta, con-
duit 24 34

26 Voltage reffulaiion 20 2

** ** of rotary con-.

23 verters ... 26 32
Voltmeter. Compensating 25 23

2 *' connections 25 22

1 •• " 26 46

** for synchronizins:. Use
28 of 23 62

24 Volume of current in stator and

rotor of induction motors .... 22 89
8

28 W

26 Wattmeter. Checking a Stanley . . 26 82
" Checkinsr of Thomson

28 recordinsr 26 79

*' Connections for Stan-

57 ley induction .... 26 77

" Connections for

15 Thomson recording: 26 77

Fort Wayne induction 26 57

13 •• Stanley induction ... 26 68

on two-phase circuit . 26 62

Thomson recordinsr . 25 69

17 •• *• " . 28 64

** with resistance. Use of

62 single 26 65

Wattmeters, Example of use of

59 three 26 63

** for unbalanced poly-

9 phase circuits, In-

59 duction 26 60

26 " Indicating: 26 64

Induction 26 64

15 ** Installation of re-

85 cording 26 76

** on three-phase cir-

66 cults 26 66

7 ** Reading: recording: . 26 82

37 " Recording 25 69

23 •• ** 26 64

Testing and adjust-
ing recording ... 26 79
Weather-proof feed-wire. Standard 24 8
** wire. Approximate

60 weights of .... 24 6
1 Weights, dimensions, etc. of bare

82 copper wire 24 4

53 Westinghouse arrester 25 52

" arrester for alter-

67 natlng current . . 25 54

Digitized by VjOOQIC

INDEX

Sec. Page
WestiDShouse arrester for hiffh-

tension lines ... 25 66
** low-eqtilvalent ar-
rester 25 57

•• plunsrer switch ... 25 7
Ume-Iimit relay . . 26 2S
Winding and connections. Field . . 22 55
** *' insolation of coils . . 22 8
'* Calculation of separately-
excited 21 84

** Compound or series-field 21 88
** for three-phase alter-
nator. Armature .... 21 15
'* for two-phase alternator.

Armature 21 18

** of induction motors. Pri-
mary 22 83

" of induction motors.

Secondary 22 85

•* Primary 22 48

** Secondary 22 50

Sinsrle-phase concen-
trated 20 22

Windings. Armature 20 21

Arransrement of .... 20 29

•* Induction-motor .... 22 88

*• Polyphase armature . . 20 27

Sec, Poet
Wire, Approximate weiirbts of

weather-proof 34 6

•• Copper 24 1

** Deflection and tensions for

aluminum 24 27

Dimensions and resistance

of iron ... 24 12
'* ** weisrhts. etc. of

bare copper . 24 4

** for a ^ven transmission . . 23 9
" Formulas for determininsr

resistance of 24 11

Srauffes 24 8

" German-silver 24 12

" Iron 24 11

Resistance, tensile strensrtb.
and weiffht of aluminum

line 24 9

*' Resistances of pure alumi-
num 24 10

" Steel 24 11

Strinsrins: aluminum .... 24 26

Table of German-silver . . 24 IS

Wires. Bare and insulated .... 24 1

Pressure 24 47

25 23

Sectional area of B. ft S. . 23 10

Digitized by VjOOQIC

168 »m ■■pll

■=" Digitized by CjOOg IC

Digitized by VjOOQIC

be dialled
t overtime.

Digitized by VjOOQIC

OCMOO

K.F. WENDTL'---" ■ •'

UWCOLLFOrCF (.■"~"

215 N. R/^Nf'Ai-L A^T-*"'[:

Digitized by VjOOQIC

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Full text of "Design of alternating current apparatus ; Electric transmission ; Line construction ; Switchboards and switchboard appliances ; Power transformation and measurement"

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